Cartesian Therapeutics (RNAC) 10-K 12 Mar 21 2020 FY Annual report Financial data

Cover page

Cover page - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 05, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-37798
Entity Registrant Name	Selecta Biosciences, Inc
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1622110
Entity Address, Address Line One	65 Grove Street,
Entity Address, City or Town	Watertown,
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02472
City Area Code	617
Local Phone Number	923-1400
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	SELB
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Smaller Reporting Company	true
Entity Emerging Growth Company	true
Entity Extended Transition Period	true
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Public Float			$ 247,960,409
Entity Common Stock, Shares Outstanding		112,373,199
Documents Incorporated by Reference	Portions of the registrant’s definitive Proxy Statement relating to its 2021 Annual Meeting of Stockholders to be filed with the Securities and Exchange Commission are incorporated by reference into Part III of this Annual Report on Form 10-K.
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001453687

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 138,685	$ 89,893
Restricted cash	0	279
Accounts receivable	7,224	5,000
Prepaid expenses and other current assets	5,434	1,495
Total current assets	151,343	96,667
Property and equipment, net	1,395	1,222
Right-of-use asset, net	10,948	301
Long-term restricted cash	1,379	1,379
Other assets	370	0
Total assets	165,435	99,569
Current liabilities:
Accounts payable	443	500
Accrued expenses	8,146	13,492
Loan payable	0	18,905
Lease liability	908	372
Deferred revenue	72,050	1,674
Total current liabilities	81,547	34,943
Non-current liabilities:
Loan payable, net of current portion	24,793	0
Lease liability	9,647	0
Deferred revenue	38,746	14,680
Warrant liabilities	28,708	41,549
Total liabilities	183,441	91,172
Commitments and contingencies (Note 17)
Stockholders’ (deficit) equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at December 31, 2020 and December 31, 2019, respectively	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 108,071,249 and 86,325,547 shares issued and outstanding as of December 31, 2020 and December 31, 2019, respectively	11	9
Additional paid-in capital	391,175	348,664
Accumulated deficit	(404,629)	(335,753)
Accumulated other comprehensive loss	(4,563)	(4,523)
Total stockholders’ (deficit) equity	(18,006)	8,397
Total liabilities and stockholders’ (deficit) equity	$ 165,435	$ 99,569

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued	108,071,249	86,325,547
Common stock, shares outstanding	108,071,249	86,325,547

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 16,597	$ 6,677	$ 903
Operating expenses:
Research and development	54,505	42,743	47,687
General and administrative	18,913	16,389	18,238
Total operating expenses	73,418	59,132	65,925
Loss from operations	(56,821)	(52,455)	(65,022)
Investment income	260	834	1,050
Loss on extinguishment of debt	(461)	0	0
Foreign currency transaction gain (loss), net	56	(47)	120
Interest expense	(1,556)	(1,519)	(1,494)
Change in fair value of warrant liabilities	(10,443)	(857)	0
Other income (expense), net	89	(1,306)	10
Net loss	(68,876)	(55,350)	(65,336)
Other comprehensive loss:
Foreign currency translation adjustment	(40)	34	(153)
Unrealized gain on securities	0	0	16
Total comprehensive loss	$ (68,916)	$ (55,316)	$ (65,473)
Net loss per share:
Basic and diluted (in dollars per share)	$ (0.68)	$ (1.22)	$ (2.92)
Weighted average common shares outstanding:
Basic and diluted (in shares)	101,202,176	45,548,511	22,389,286

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders’ (Deficit) Equity - USD ($) $ in Thousands	Total	Private Placement	Cumulative Effect, Period of Adoption, Adjustment	Common stock	Common stockPrivate Placement	Additional paid-in capital	Additional paid-in capitalPrivate Placement	Accumulated deficit	Accumulated deficitCumulative Effect, Period of Adoption, Adjustment	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2017				22,343,254
Beginning balance at Dec. 31, 2017	$ 51,814		$ 1,830	$ 3		$ 273,128		$ (216,897)	$ 1,830	$ (4,420)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)				24,738
Issuance of common stock under Employee Stock Purchase Plan	196					196
Issuance of common stock upon exercise of options (in shares)				103,784
Issuance of common stock upon exercise of options	501					501
Stock-based compensation expense	5,714					5,714
Currency translation adjustment	(153)									(153)
Unrealized gain on securities	16									16
Net loss	(65,336)							(65,336)
Ending balance (in shares) at Dec. 31, 2018				22,471,776
Ending balance at Dec. 31, 2018	(5,418)			$ 3		279,539		(280,403)		(4,557)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)				17,205
Issuance of common stock under Employee Stock Purchase Plan	28					28
Issuance of common stock upon exercise of options (in shares)				125,600
Issuance of common stock upon exercise of options	150					150
Issuance of vested restricted stock units (in shares)				93,750
Issuance of vested restricted stock units	0
Issuance of common stock, net of issuance costs (in shares)				22,188,706	3,178,174
Issuance of common stock, net	30,942	$ 5,715		$ 2		30,940	$ 5,715
Issuance of common stock through at-the-market offering, net (in shares)				615,453
Issuance of common stock through at-the-market offering, net	1,006					1,006
Issuance of common stock, pre-funded warrants and warrants through private placement (in shares)				37,634,883
Issuance of common stock, pre-funded warrants and warrants through private placement	26,129			$ 4		26,125
Stock-based compensation expense	5,161					5,161
Currency translation adjustment	34									34
Unrealized gain on securities	0
Net loss	$ (55,350)							(55,350)
Ending balance (in shares) at Dec. 31, 2019	86,325,547			86,325,547
Ending balance at Dec. 31, 2019	$ 8,397			$ 9		348,664		(335,753)		(4,523)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)				110,212
Issuance of common stock under Employee Stock Purchase Plan	184					184
Issuance of common stock upon exercise of options (in shares)				76,128
Issuance of common stock upon exercise of options	193					193
Issuance of vested restricted stock units (in shares)				93,750
Issuance of vested restricted stock units	0
Issuance of common stock, net of issuance costs (in shares)					5,416,390
Issuance of common stock, net		$ 10,268					$ 10,268
Issuance of common stock through at-the-market offering, net (in shares)				1,069,486
Issuance of common stock through at-the-market offering, net	2,108					2,108
Issuance of common stock upon exercise of pre-funded warrants (in shares)				8,342,128
Issuance of common stock upon exercise of pre-funded warrants	1			$ 1
Issuance of common stock upon exercise of common warrants (in shares)				6,637,608
Issuance of common stock upon exercise of common warrants	24,263			$ 1		24,262
Other financing fees	(370)					(370)
Issuance of common warrants with long-term debt, net	444					444
Stock-based compensation expense	5,422					5,422
Currency translation adjustment	(40)									(40)
Unrealized gain on securities	0
Net loss	$ (68,876)							(68,876)
Ending balance (in shares) at Dec. 31, 2020	108,071,249			108,071,249
Ending balance at Dec. 31, 2020	$ (18,006)			$ 11		$ 391,175		$ (404,629)		$ (4,563)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (68,876)	$ (55,350)	$ (65,336)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	734	726	975
Accretion of discounts on investments	0	(154)	(101)
Non-cash lease expense	1,127	1,301	0
(Gain) loss on disposal of property and equipment	(52)	104	(81)
Stock-based compensation expense	5,422	5,161	5,714
Non-cash interest expense	620	402	449
Warrant liabilities revaluation	10,443	857	0
Loss on extinguishment of debt	461	0	0
Net realized losses on investments	0	(1)	0
Changes in operating assets and liabilities:
Accounts receivable	(2,224)	(5,000)	0
Prepaid expenses, deposits and other assets	(4,418)	3,179	(2,631)
Accounts payable	(57)	(600)	(516)
Deferred revenue	94,462	337	(101)
Accrued expenses and other liabilities	(2,761)	(2,397)	2,467
Net cash provided by (used in) operating activities	34,881	(51,435)	(59,161)
Cash flows from investing activities
Receipts from the maturity of short-term investments	0	16,350	41,655
Purchases of short-term investments	0	(18,188)	(15,598)
Sale of short term investments	0	1,992	0
Purchases of property and equipment	(815)	(47)	(884)
Proceeds from the sale of property and equipment	74	122	99
Net cash (used in) provided by investing activities	(741)	229	25,272
Cash flows from financing activities
Proceeds from issuance of long-term debt, net of expenses	24,736	0	0
Repayments of principal on outstanding debt	(19,313)	(2,800)	0
Issuance costs paid for December 2019 financing	(4,381)	0	0
Other financing fees	(343)	0	0
Net proceeds from exercise of pre-funded and common warrants	979	70,000	0
Proceeds from exercise of stock options	193	150	501
Proceeds from issuance of common stock under Employee Stock Purchase Plan	184	28	196
Net cash provided by financing activities	14,431	105,041	697
Effect of exchange rate changes on cash	(58)	34	(153)
Net change in cash, cash equivalents, and restricted cash	48,513	53,869	(33,345)
Cash, cash equivalents, and restricted cash at beginning of period	91,551	37,682	71,027
Cash, cash equivalents, and restricted cash at end of period	140,064	91,551	37,682
Supplement cash flow information
Cash paid for interest	1,018	1,223	1,134
Noncash investing and financing activities
Cashless warrant exercise	21,790	0	0
Reclassification of warrant liability to equity upon exercise of warrants	1,494	0	0
Fair value of warrants issued in connection with issuance of long-term debt	444	0	0
Purchase of property and equipment not yet paid	4	0	145
Equity offering costs in accrued liabilities	27	4,381	0
Unrealized gain on marketable securities	0	0	16
Debt issuance costs in accrued liabilities	2	0	0
Stock Offering, Excluding At The Market Offering And Private Placement
Cash flows from financing activities
Net proceeds from issuance of common stock	0	30,942	0
At-The-Market Offering
Cash flows from financing activities
Net proceeds from issuance of common stock	2,108	1,006	0
Private Placement
Cash flows from financing activities
Net proceeds from issuance of common stock	$ 10,268	$ 5,715	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Selecta Biosciences, Inc. (the “Company”) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a clinical-stage biopharmaceutical company leveraging its ImmTOR™ immune tolerance platform with the goals of amplifying the efficacy of biologics, including enabling the re-dosing of life-saving gene therapies, and restoring self-tolerance in autoimmune diseases. The Company's ImmTOR platform encapsulates rapamycin, also known as sirolimus, an immunomodulator, in biodegradable nanoparticles and is designed to induce antigen-specific immune tolerance. The Company believes ImmTOR has the potential to enhance the efficacy without compromising the safety of biologic therapies, improve product candidates under development, and enable novel therapeutic modalities. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Liquidity and Management's Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through the initial public offering of its common stock, private placements of its common stock, issuances of common and preferred stock, debt, research grants and research collaborations. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, all of the Company's revenue has been collaboration and grant revenue. The Company has devoted substantially all of its financial resources and efforts to developing its ImmTOR platform, identifying potential product candidates and conducting preclinical studies and its clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any ImmTOR-enabled therapies. As of December 31, 2020, the Company’s cash, cash equivalents and restricted cash were $140.1 million, of which $1.4 million was restricted cash related to lease commitments and $0.3 million was held by its Russian subsidiary designated solely for use in its operations. The Company believes the cash, cash equivalents, and restricted cash as of December 31, 2020 will enable us to fund our operating expenses and capital expenditure requirements into the second quarter of 2023. The Company has incurred losses and negative cash flows from operating activities since inception. As of December 31, 2020, the Company had an accumulated deficit of $404.6 million. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research, development of its product candidates, conducting preclinical studies and clinical trials, and its administrative organization. The Company will require substantial additional financing to fund its operations and to continue to execute its strategy, and the Company will pursue a range of options to secure additional capital. At this time, there is significant uncertainty relating to the trajectory of the COVID-19 pandemic and the impact of related responses. Any impact of COVID-19 on our business, revenues, results of operations and financial condition will largely depend on future developments, which are highly uncertain and cannot be predicted with confidence, such as the ultimate geographic spread of the disease, the duration of the pandemic, travel restrictions and social distancing in the United States and other countries, business closures or business disruptions, the ultimate impact on financial markets and the global economy, and the effectiveness of actions taken in the United States and other countries to contain and treat the disease. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2020, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta (RUS), LLC, or Selecta (RUS), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated. Foreign Currency The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ (deficit) equity. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, the valuation of its warrant liabilities and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Cash Equivalents, Investments and Restricted Cash Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these marketable securities and records them at fair value in the accompanying consolidated balance sheets. Investments with less than one year until maturity are classified as short term, while investments with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the year ended December 31, 2020, there were no realized losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value. As of December 31, 2020, the Company had restricted cash balances relating to a secured letter of credit in connection with its Headquarters Lease (as defined in Note 8). The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows: Year Ended December 31, 2020 2019 2018 Cash and cash equivalents $ 138,685 $ 89,893 $ 37,403 Short-term restricted cash — 279 — Long-term restricted cash 1,379 1,379 279 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 140,064 $ 91,551 $ 37,682 Concentrations of Credit Risk and Off-Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially creditworthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2020, the Company maintained approximately $0.3 million in Russian bank accounts, all of which was held in U.S. dollars. The Company did not have any off-balance sheet arrangements as of December 31, 2020 and December 31, 2019. Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, and common warrants. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2020, given the recent issuance of the Term A Loan under the 2020 Term Loan (each, as defined below), the Company believes the carrying value approximates the fair value of the loan. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three-level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of warrant liabilities were determined using Level 3 inputs. Fair value is a market-based measure considered from the perspective of a market participant rather than an entity-specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the years ended December 31, 2020 or December 31, 2019. Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use. Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available, which is at the entity level (“asset group”). An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. Based on management's evaluation, the fair value of the asset group, measured as the market capitalization of the Company exceeds its carrying value, and for this reason the Company did not recognize any material impairment losses during the years ended December 31, 2020 and 2019. Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the effective interest method and recorded as interest expense. Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non-owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments. Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC Topic 606, Revenue from Contracts with Customers (ASC 606) , a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Swedish Orphan Biovitrum (“Sobi”), Asklepios Biopharmaceutical, Inc. (“AskBio”), Sarepta Therapeutics, Inc. (“Sarepta”), and Spark Therapeutics, Inc. (“Spark”) (see Note 12) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue: The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Grants and license agreements with customers are accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808 ), and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). The Company early adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which provides guidance on evaluating certain transactions between collaborative arrangement participants. If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC Topic 730, Research and Development (ASC 730) , and records reimbursements from counterparties as an offset to the related costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) up-front fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of research and development (R&D) expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint. Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, stock-based compensation expenses, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancellable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs. On June 11, 2020, we and Sobi entered into a license and development agreement (the “Sobi License”). Pursuant to the Sobi License, clinical trial costs incurred to complete development of SEL-212, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, will be reimbursed by Sobi. These costs, when reimbursed, will be recognized as revenue consistent with the revenue recognition methodology disclosed in Footnote 12. The reimbursable costs exclude any costs of additional development activities required that are related to ImmTOR and that are unrelated to SEL-212. Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions. Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity , and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock . Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date. Stock-Based Compensation The Company accounts for all stock-based compensation granted to employees and non-employees using a fair value method. Stock-based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis, net of estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock-based compensation expense recognized in the consolidated financial statements is based on awards that ultimately vest. Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares and pre-funded warrants outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and basic and diluted loss per share have been the same. Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies . A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. Leases The Company accounts for its leases in accordance with ASC Topic 842, Leases (ASC 842) , and determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company elected not to recognize leases with a term less than one year on its balance sheet. Operating lease right-of-use (ROU) assets and their corresponding lease liabilities are recorded based on the present value of lease payments over the expected remaining lease term. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. In accordance with the guidance in ASC 842, components of a lease should be split into three categories: lease components (e.g. land, building, etc.), non-lease components (e.g. common area maintenance, consumables, etc.), and non-components (e.g. property taxes, insurance, etc.). Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, the Company elected the practical expedient to not separate lease and non-lease components. The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. Right-of-use assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and estimated incremental borrowing rate upon lease modification. See Note 8 for details. Recent Accounting Pronouncements Recently Adopted In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13) which changes the fair value measurement disclosure requirements of ASC 820. Entities will no longer be required to disclose the amount of, and reasons for, transfers between Level 1 and Level 2 of the fair value hierarchy, the policy of timing of transfers between levels of the fair value hierarchy and the valuation processes for Level 3 fair value measurements. The Company adopted the new standard effective January 1, 2020, and there was no impact on its consolidated financial statements. Not Yet Adopted In October 2020, the FASB issued ASU 2020-10, Codification Improvements , which updates various codification topics by clarifying or improving disclosure requirements to align with the SEC’s regulations. This ASU is effective for public entities for fiscal years beginning after December 15, 2020. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815 – 40) . ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2023, with early adoption permitted no earlier than January 1, 2021. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740) - Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. This ASU is effective for pu

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	Marketable SecuritiesAs of December 31, 2020, and December 31, 2019, the Company did not have marketable securities.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The Company has reported a net loss for the years ended December 31, 2020, 2019 and 2018. For this reason basic and diluted net loss per share are the same for all periods presented. Since the shares underlying the 8,342,128 pre-funded warrants were issuable for little or no consideration, they were considered outstanding for both basic and diluted earnings per share. During the second quarter 2020, all 8,342,128 pre-funded warrants were exercised, but had no effect on basic and diluted shares at exercise because all were included in both basic and diluted from the period of issuance. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per-share data): Year Ended December 31, 2020 2019 2018 Numerator: Net loss attributable to common stockholders $ (68,876) $ (55,350) $ (65,336) Denominator: Weighted-average common shares and pre-funded warrants outstanding—basic and diluted 101,202,176 45,548,511 22,389,286 Net loss per share attributable to common stockholders —basic and diluted $ (0.68) $ (1.22) $ (2.92) All potential dilutive common shares have been excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive. Potential dilutive common share equivalents consist of the following: Year Ended December 31, 2020 2019 2018 Stock options to purchase common stock 7,775,249 6,796,669 4,093,979 Unvested restricted stock units 87,500 181,250 175,000 Stock warrants to purchase common stock 12,378,016 23,084,120 95,619 Total 20,240,765 30,062,039 4,364,598

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Assets and Liabilities Measured at Fair Value on a Recurring Basis The tables below present information about the Company’s financial assets and liabilities that are measured and carried at fair value as of December 31, 2020 and December 31, 2019, and indicate the level within the fair value hierarchy where each measurement is classified. Below is a summary of assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2020 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 80,576 $ 80,576 $ — $ — Total $ 80,576 $ 80,576 $ — $ — Liabilities: Warrant liabilities $ 28,708 $ — $ — $ 28,708 Total $ 28,708 $ — $ — $ 28,708 December 31, 2019 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 50,401 $ 50,401 $ — $ — Total $ 50,401 $ 50,401 $ — $ — Liabilities: Warrant liabilities $ 41,549 $ — $ — $ 41,549 Total $ 41,549 $ — $ — $ 41,549 As of December 31, 2020 and December 31, 2019, the money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase. Assumptions Used in Determining Fair Value of Common Warrants In December 2019, the Company issued common warrants in connection with a private placement of common shares. Pursuant to the terms of the common warrants, the Company could be required to settle the common warrants in cash in the event of certain acquisitions of the Company and, as a result, the common warrants are required to be measured at fair value and reported as a liability on the balance sheet. The Company recorded the fair value of the common warrants upon issuance using the Black-Scholes valuation model and is required to revalue the common warrants at each reporting date with any changes in fair value recorded in the statement of operations. The valuation of the common warrants is considered under Level 3 of the fair value hierarchy due to the need to use assumptions in the valuation that are both significant to the fair value measurement and unobservable. The significant unobservable inputs used in the fair value measurement of the warrant liabilities were the volatility rate and the estimated term of the warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The change in the fair value of the Level 3 warrant liability is reflected in the statement of operations for the year ended December 31, 2020. The estimated fair value of warrants is determined using Level 3 inputs inherent in the Black-Scholes simulation valuation. Estimated fair value of the underlying stock . The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period. Risk-free interest rate . The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption. Dividend rate . The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. Expected life . The expected life of the warrants is assumed to be equivalent to their remaining contractual term which expires on December 23, 2024. Volatility . The Company estimates stock price volatility based on the Company’s historical volatility and the historical volatility of peer companies for a period of time commensurate with the expected remaining life of the warrants. A summary of the Black-Scholes pricing model assumptions used to record the fair value of the warrant liability is as follows: December 31, 2020 Risk-free interest rate 0.36 % Dividend yield — Expected life (in years) 3.98 Expected volatility 98.63 % Changes in Level 3 Liabilities Measured at Fair Value on a Recurring Basis The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 10), for the year ended December 31, 2020 (in thousands): Warrant liabilities Fair value as of December 31, 2019 $ 41,549 Exercises (23,284) Change in fair value 10,443 Fair value as of December 31, 2020 $ 28,708

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): December 31, December 31, 2020 2019 Laboratory equipment $ 4,427 $ 4,836 Computer equipment and software 532 515 Leasehold improvements 38 278 Furniture and fixtures 327 237 Office equipment 163 135 Construction in process 163 2 Total property and equipment 5,650 6,003 Less accumulated depreciation (4,255) (4,781) Property and equipment, net $ 1,395 $ 1,222 Depreciation expense was $0.6 million, $0.7 million and $1.0 million for the years ended December 31, 2020, 2019 and 2018, respectively. During the fourth quarter of 2020, capitalized website development costs of $0.1 million were recorded within construction in process. The Company recorded accelerated depreciation costs of less than $0.1 million in the reported property and equipment for the year ended December 31, 2020 relating to the new corporate headquarters move in 2020.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): December 31, December 31, 2020 2019 Payroll and employee related expenses $ 3,049 $ 2,235 Collaboration and licensing 1,350 1,050 Accrued patent fees 534 487 Accrued external research and development costs 2,029 4,379 Accrued professional and consulting services 798 468 Accrued interest 170 82 Issuance costs, December 2019 financing — 4,381 Other 216 410 Accrued expenses $ 8,146 $ 13,492

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	Leases On January 1, 2019, the Company adopted ASC 842 using the modified retrospective approach. The Company recorded operating lease assets (right-of-use assets) of $1.6 million and operating lease liabilities of $1.8 million and reversed a lease liability of $0.2 million related to straight-line rent and incentives. There was no impact to accumulated deficit upon adoption of ASC 842. The underlying assets of the Company’s leases are primarily office space. The Company determines if an arrangement qualifies as a lease at its inception. As a practical expedient permitted under ASC 842, the Company elected to account for the lease and non-lease components as a single lease component for all leases of which it is the lessee. Lease payments, which may include lease and non-lease components, were included in the measurement of the Company’s lease liabilities to the extent that such payments were either fixed amounts or variable amounts dependent upon on a rate or index as stipulated in the lease contract. When the Company cannot readily determine the rate implicit in the lease, the Company determines its incremental borrowing rate by using the rate of interest that it would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. On January 1, 2019, the discount rate used on existing operating leases at adoption, which had remaining lease terms of 15 months, was 10.0%. The Company enters into lease agreements with terms generally ranging from 2-8 years. Some of the Company’s lease agreements include Company options to either extend and/or early terminate the lease, the costs of which are included in its operating lease liabilities to the extent that such options are reasonably certain of being exercised. Leases with renewal options allow the Company to extend the lease term typically between 1 and 5 years. When determining the lease term, renewal options reasonably certain of being exercised are included in the lease term. When determining if a renewal option is reasonably certain of being exercised, the Company considers several economic factors, including but not limited to, the significance of leasehold improvements incurred on the property, whether the asset is difficult to replace, underlying contractual obligations, or specific characteristics unique to that particular lease that would make it reasonably certain that the Company would exercise such option. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases. Leases with an initial term of 12 months or less are not recorded on the balance sheet; the Company recognizes lease expense for these leases on a straight-line basis over the lease term. 480 Arsenal Way Lease The Company had a non-cancellable operating lease for its laboratory and office space located at 480 Arsenal Way, Watertown, Massachusetts, or the Prior Headquarters Lease. Pursuant to the Prior Headquarters Lease, the landlord provided the Company a tenant improvement allowance of up to $0.7 million, which the Company fully utilized during 2012. The leasehold improvements were capitalized as a component of property and equipment. In connection with the Prior Headquarters Lease, the Company had a letter of credit for $0.3 million which renewed automatically each year and was classified in restricted cash. In August 2016, the Company signed an amendment to the Prior Headquarters Lease, which extended the term through March 31, 2020. In March 2020, the Company signed an amendment to extend the lease term one additional month to April 30, 2020. The right-of-use asset and lease liability were remeasured and recorded based on the change in the lease term in which the net impact was immaterial. 75 North Beacon Street Lease In October 2017, the Company entered into a lease for approximately 5,100 square feet of additional office space located at 75 North Beacon Street, Watertown, Massachusetts, or the 75 North Beacon Lease for a term through March 31, 2020. On January 11, 2019, the Company vacated 75 North Beacon Street, Watertown, MA and consolidated all employees at its then- corporate headquarters at 480 Arsenal Way, Watertown, MA. The right-of-use asset with carrying amount of $0.2 million attributable to the 75 North Beacon Lease was written down to zero during the first quarter of 2019. 65 Grove Street Lease In July 2019, the Company entered into a lease for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts, or the Headquarters Lease. As part of the Headquarters Lease, the Company incurred $0.8 million in non-reimbursable construction costs. The lease began in March 2020, consistent with when the Company took control of the office space and the lease term is 8 years. The discount rate of 8.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods. Rent payments began in May 2020, and the base rent for the first year is $0.2 million per month. In connection with the Headquarters Lease, the Company secured a letter of credit from Silicon Valley Bank for $1.4 million which renews automatically each year. The Company recorded the right-of-use asset and operating lease liabilities of $11.8 million during the three months ended March 31, 2020 as control of the premises was transferred to the Company. Moscow, Russia Lease The Company has a month-to-month facility agreement for its Moscow, Russia office. Rent expense is recognized as incurred. Rent expense for the years ended December 31, 2020, 2019 and 2018 was $2.7 million, $2.1 million and $2.0 million, respectively. As of December 31, 2020 and 2019, the components of the operating leases were as follows (in thousands): December 31, 2020 2019 Assets: Right-of-use asset, net $ 10,948 $ 301 Liabilities: Current operating lease liabilities $ 908 $ 372 Non-current operating lease liabilities 9,647 — Total operating lease liabilities $ 10,555 $ 372 For the years ended December 31, 2020 and 2019 the components of lease costs were as follows (in thousands): Year Ended December 31, 2020 2019 Operating lease cost $ 2,096 $ 1,365 Variable lease cost 624 828 Short-term lease cost 10 16 Total lease cost $ 2,730 $ 2,209 The maturity of the Company's operating lease liabilities as of December 31, 2020 were as follows (in thousands): December 31, 2020 2021 $ 1,812 2022 1,866 2023 1,922 2024 1,980 2025 2,039 Thereafter 4,945 Total future minimum lease payments $ 14,564 Less imputed interest 4,009 Total operating lease liabilities $ 10,555 The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of lease liabilities: $ 2,523 $ 1,482 Other than the initial recording of the right-of-use asset and lease liability for the Headquarters Lease, which is non-cash, the changes in the Company’s right-of-use asset and lease liability for the years ended December 31, 2020 and 2019 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows. The following summarizes additional information related to operating leases: December 31, 2020 2019 Weighted-average remaining lease term 7.4 years 0.3 years Weighted-average discount rate 8.9 % 10 %

Debt

Debt	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt	Debt 2020 Term Loan On August 31, 2020, the Company entered into a term loan of up to $35.0 million (the “2020 Term Loan”), consisting of term loans in an aggregate amount of $25.0 million (the “Term A Loan”) and term loans in an aggregate amount of $10.0 million (the “Term B Loan”), governed by a loan and security agreement between the Company and Oxford Finance LLC, a Delaware limited liability company (“Oxford”) as Collateral Agent and a Lender, and Silicon Valley Bank, a California corporation (“SVB”), as a Lender (the “Loan Agreement”). The Term A Loan was funded in full on August 31, 2020 (the “Funding Date”). The Term B Loan will be available, subject to Collateral Agent’s discretion and customary terms and conditions, during the period commencing on the date the Company has delivered to the Collateral Agent and the Lenders evidence: (i) the Company or one of the Company’s collaboration partners has enrolled its first randomized patient for a Phase 1 clinical trial evaluating the treatment of methylmalonic acidemia (“MMA”), and (ii) the Company has enrolled the first patient in each of two Phase 3 pivotal trials evaluating SEL-212 (the “Second Draw Period Milestone”) and ending on the earliest of (i) the date which is thirty (30) days following the date the Second Draw Period Milestone is achieved, (ii) September 30, 2021 (iii) and the occurrence of an event of default, other than an event of default that has been waived in writing by Collateral Agent and the Lenders in their sole discretion (such period, the “Second Draw Period”). The 2020 Term Loan will mature on August 1, 2025. Each advance under the Term Loan accrues interest at a floating per annum rate equal to the greater of (a) 7.90%, and (b) the lesser of (x) the sum of (i) the prime rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, and (ii) 4.65% and (y) 10.00%. The Term Loan provides for interest-only payments on a monthly basis until April 1, 2022; provided however, if the Company has delivered to the Collateral Agent and the Lenders prior to September 30, 2021 evidence that Borrower has achieved the Second Draw Period Milestone, the Term Loan provides for interest-only payments on a monthly basis until October 1, 2022. Thereafter, amortization payments will be payable monthly in equal installments of principal and interest to fully amortize the outstanding principal over the remaining term of the loan, subject to recalculation upon a change in the prime rate. The Company may prepay the Term Loan in full but not in part provided that the Company (i) provides ten days’ prior written notice to Collateral Agent, (ii) pays on the date of such prepayment (A) all outstanding principal plus accrued and unpaid interest, and (B) a prepayment fee of between 3.0% and 1.0% of the aggregate original principal amount advanced by the lender depending on the timing of the prepayment. Amounts outstanding during an event of default are payable upon SVB’s demand and shall accrue interest at an additional rate of 5.0% per annum of the past due amount outstanding. At the end of the loan term (whether at maturity, by prepayment in full or otherwise), the Company shall make a final payment to the lender in the amount of 9.0% of the aggregate original principal amount advanced by the lender. The final payment fee totaling $2.3 million is recorded as a loan discount. The Term Loan is secured by a lien on substantially all of the assets of the Company, other than intellectual property, provided that such lien on substantially all assets includes any rights to payments and proceeds from the sale, licensing or disposition of intellectual property. The Company has also granted the Collateral Agent a negative pledge with respect to its intellectual property. The Loan Agreement contains customary covenants and representations, including but not limited to financial reporting obligations and limitations on dividends, indebtedness, collateral, investments, distributions, transfers, mergers or acquisitions, taxes, corporate changes, deposit accounts, and subsidiaries. The Loan Agreement also contains other customary provisions, such as expense reimbursement, non-disclosure obligations as well as indemnification rights for the benefit of the Collateral Agent. The events of default under the Loan Agreement include, but are not limited to, the Company’s failure to make any payments of principal or interest under the Loan Agreement or other transaction documents, the Company’s breach or default in the performance of any covenant under the Loan Agreement or other transaction documents, the occurrence of a material adverse change, the Company making a false or misleading representation or warranty in any material respect under the Loan Agreement, the Company’s insolvency or bankruptcy, any attachment or judgment on the Company’s assets of at least $0.5 million, or the occurrence of any default under any agreement or obligation of the Company involving indebtedness in excess of $0.5 million. If an event of default occurs, the Collateral Agent is entitled to take enforcement action, including acceleration of amounts due under the Loan Agreement. The Company incurred $0.4 million in debt issuance costs in connection with the closing of the 2020 Term Loan. Debt issuance costs are presented in the consolidated balance sheet as a direct deduction from the associated liability and amortized to interest expense over the term of the related debt. The Company assessed all terms and features of the 2020 Term Loan to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the 2020 Term Loan, including any put, call, and contingent features. The Company determined that the interest rate collar and prepayment call option did not require bifurcation; whereas the contingent put option and default (contingent) interest rate feature met bifurcation criteria resulting in immaterial amounts. Warrants On August 31, 2020, in connection with the 2020 Term A Loan, the Company issued warrants to the Lenders to purchase an aggregate of 196,850 shares of its common stock at an exercise price equal to $2.54 per share. In accordance with ASC 815-40, these warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire ten years from the date of issuance. The initial grant date fair value of the warrants was $0.4 million as determined by the Black-Scholes valuation model and recorded to shareholders' equity, with the SVB portion allocated to the reacquisition price of the 2017 Term Loan and the Oxford fair value portion as a loan discount to the Term A Loan. Additionally, on August 31, 2020, pursuant to the terms of a Warrant Side Letter agreement among the Company and the Lenders, the Company agreed to issue to the Lenders, on the date the Company draws the Term B Loan and in accordance with each party’s respective pro rata share with respect to the Term B Loan, one or more warrants to purchase an aggregate number of shares of its common stock that is equal to $200,000 divided by the average closing price of the Company’s common stock on The Nasdaq Stock Market LLC for the ten consecutive trading days ending the day before such issuance, rounded down to the nearest whole number of shares, and having an exercise price equal to the Term B Warrant Price. Payoff On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which the Company utilized $13.7 million of the 2020 Term Loan to pay off all outstanding obligations under the 2017 Term Loan (as defined below), consisting of the principal payment, final prepayment and accrued interest. During the three months ended September 30, 2020, the Company recognized a loss on extinguishment of debt in the amount of $0.5 million determined as the difference between the reacquisition price and carrying value at August 31, 2020. 2017 Term Loan On September 12, 2017, the Company entered into a term loan of up to $21.0 million (the “2017 Term Loan” and, together with the 2020 Term Loan, the “Term Loans”)) with Silicon Valley Bank. The 2017 Term Loan was governed by a loan and security agreement, dated September 12, 2017, between the Company and SVB. The 2017 Term Loan was funded in full on September 13, 2017. The Company incurred less than $0.1 million in debt issuance costs in connection with the closing of the 2017 Term Loan. Debt issuance costs were presented in the consolidated balance sheet as a direct deduction from the associated liability and amortized to interest expense over the term of the related debt. Prior to the extinguishment of the 2017 Term Loan in August 2020 discussed in greater detail above, the 2017 Term Loan was secured by a lien on substantially all of the assets of the Company, other than intellectual property, provided that such lien on substantially all assets included any rights to payments and proceeds from the sale, licensing or disposition of intellectual property. The Company had also granted SVB a negative pledge with respect to its intellectual property. Each advance under the 2017 Term Loan accrued interest at a floating per annum rate equal to one-half of one percent above the prime rate (as published in the money rates section of The Wall Street Journal). The 2017 Term Loan provided for interest-only payments monthly through August 31, 2019. The monthly interest was subject to recalculation upon a change in the prime rate. As of December 31, 2020, the outstanding principal balance was $25.0 million under the 2020 Term Loan and as of December 31, 2019, the outstanding principal balance was $18.2 million under the 2017 Term Loan. Future minimum principal and interest payments on the 2020 Term Loan as of December 31, 2020 are as follows (in thousands): 2021 2,002 2022 7,343 2023 8,611 2024 8,027 2025 7,274 Total minimum debt payments $ 33,257 Less: Amount representing interest (6,008) Less: Debt discount and deferred charges (2,456) Less: Current portion of loan payable — Loan payable, net of current portion $ 24,793 During the years ended December 31, 2020, 2019 and 2018, the Company recognized $1.6 million, $1.5 million and $1.5 million respectively of interest expense related to the 2020 and 2017 Term Loans.

Equity

Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Equity	Equity Equity Financings August 2020 Shelf Registration Statement On August 6, 2020, the Company filed an updated universal shelf registration statement on Form S-3 (Reg. No. 333-241692) with the SEC to sell an aggregate amount of up to $200.0 million of certain of its securities. The shelf registration statement was declared effective by the SEC on August 14, 2020. “At-the-Market” Offerings In August 2017, the Company entered into a sales agreement (the “2017 Sales Agreement”), with Jefferies LLC, as sales agent, to sell shares of its common stock with an aggregate value of up to $50 million in an “at the market offering.” On August 6, 2020, concurrent with the filing of the updated shelf registration statement, the Company entered into a new sales agreement (the “2020 Sales Agreement”) with Jefferies LLC, as sales agent, pursuant to which the Company may, from time to time, issue and sell common stock with an aggregate value of up to $50 million in an “at the market offering.” The 2017 Sales Agreement terminated pursuant to its terms in August 2020. Sales of common stock, if any, pursuant to the 2020 Sales Agreement, may be made in sales deemed to be an “at the market offering” as defined in Rule 415(a) of the Securities Act, including sales made directly through the Nasdaq Stock Market or on any other existing trading market for the Company’s common stock. The Company intends to use the proceeds from the offering for working capital and other general corporate purposes. The Company may suspend or terminate the 2020 Sales Agreement at any time. During the year ended December 31, 2019, the Company sold 615,453 shares of its common stock pursuant to the 2017 Sales Agreement at an average price of approximately $1.84 per share for aggregate net proceeds of $1.0 million, after deducting commissions and other transaction costs, pursuant to its shelf registration statement filed in August 2017. During the year ended December 31, 2020, the Company sold 1,069,486 shares of its common stock pursuant to the 2017 Sales Agreement and the 2020 Sales Agreement, as applicable, at an average price of approximately $2.16 per share for aggregate net proceeds of $2.1 million, after deducting commissions and other transaction costs. June 2020 Sobi Stock Purchase On June 11, 2020, the Company entered into a stock purchase agreement with Swedish Orphan Biovitrum AB (Publ), pursuant to which the Company sold an aggregate of 5,416,390 shares of its common stock at a purchase price equal to $4.6156 per share, which represented 120% of the 10-day volume-weighted average price of the Company’s common stock prior to signing, for aggregate gross proceeds of $25 million, the Sobi Private Placement. The closing of the Sobi Private Placement occurred on July 31, 2020. The shares of common stock acquired in the Sobi Private Placement are subject to a one-year lock-up from closing, during which time Sobi is prohibited from selling or otherwise disposing of such shares. In accordance with ASC 815, this forward sale treatment qualified as equity classification as the shares are not within the scope of ASC 480. The gross proceeds of $25 million were determined to include a premium to the fair value of the Company’s shares as of July 28, 2020 of approximately $14.5 million. As a result, such amount was included in the transaction price for revenue recognition of the Sobi License. See Note 12 for details. Also on June 11, 2020, the Company entered into a registration rights agreement (as amended by that certain letter agreement, dated as of November 4, 2020, the “Sobi Registration Rights Agreement”) with Sobi, pursuant to which the Company agreed to prepare and file a registration statement with respect to the resale of the shares of common stock acquired in the Sobi Private Placement. The Company will be required to file this resale registration statement within 30 days following receipt by the Company of a written request from Sobi to file such resale registration statement, and to have the registration statement declared effective within ten (10) Business Days after the SEC informs the Company that no review of such resale registration statement will be made or that the SEC has no further comments on such resale registration statement. December 2019 Financing On December 18, 2019, the Company entered into a securities purchase agreement (the “2019 Purchase Agreement”) with a group of institutional investors and certain members of the board of directors. Pursuant to the 2019 Purchase Agreement, the Company sold an aggregate of 37,634,883 shares of its common stock at a purchase price of $1.46 per share, warrants to purchase an aggregate of 22,988,501 shares of common stock at a purchase price of $0.125 per share underlying each common warrant, and pre-funded warrants to purchase an aggregate of 8,342,128 shares of common stock at a purchase price of $1.46 per share, all with five year terms (the “2019 PIPE”). The closing of the 2019 PIPE occurred on December 23, 2019. The exercise price of the pre-funded warrants is $0.0001 per share and the exercise price for the common warrants is $1.46 per share. In the event of a certain sale of the Company, the terms of the common warrants require us to make a payment to such common warrant holders based on a Black-Scholes valuation (using variables as specified in the warrants). This provision does not apply to the pre-funded warrants. Therefore, the Company is required to account for the common warrants as liabilities and record them at fair value, while the pre-funded warrants met the criteria to be classified as permanent equity. The Company recorded the fair value of the common warrants of $40.7 million upon issuance using the Black-Scholes valuation model. Issuance costs were allocated between the equity component with an offset to additional paid-in capital and the liability component recorded as expense on a relative fair value basis. Total net proceeds from the equity offering was $65.6 million, after deducting transaction costs and commissions of $4.4 million which was paid in the three months ended March 31, 2020. The common warrants were revalued as of December 31, 2020 at $28.7 million. During the years ended December 31, 2020 and 2019, the Company recorded losses on the increases in the fair value of the warrants of $(10.4) million and $(0.9) million, respectively, in the consolidated statements of operations. On December 23, 2019, in connection with the 2019 PIPE, the Company entered into a registration rights agreement (the “2019 Registration Rights Agreement”), pursuant to which the Company agreed to prepare and file a registration statement with the SEC within 45 days after the closing of the 2019 PIPE for purposes of registering the resale of the shares of common stock issued and sold in the 2019 PIPE, shares of common stock issuable upon exercise of the warrants sold in the 2019 PIPE, and any shares of common stock issued as a dividend or other distribution with respect to the shares of common stock or shares of common stock issuable upon exercise of the warrants. The 2019 PIPE registration statement was declared effective by the SEC on February 6, 2020. The Company agreed, among other things, to indemnify the investors in the 2019 PIPE, and their officers, directors, members, employees and agents, successors and assigns, under the registration statement from certain liabilities and to pay all fees and expenses (excluding any legal fees of the selling holder(s), and any underwriting discounts and selling commissions) incident to the Company’s obligations under the 2019 Registration Rights Agreement. June 2017 Financing On June 26, 2017, the Company entered into a securities purchase agreement (the “Institutional Purchase Agreement”) with a select group of institutional investors (the “Institutional Investors”) and a securities purchase agreement with Timothy A. Springer, Ph.D., a member of the board of directors (the “Springer Purchase Agreement”) for a private placement of the Company's securities (the “2017 PIPE”). The closing of the 2017 PIPE occurred on June 27, 2017. Pursuant to the Institutional Purchase Agreement, the Company sold an aggregate of 2,750,000 shares of its common stock at a purchase price equal to $16.00 per share. Pursuant to the Springer Purchase Agreement, the Company sold to Dr. Springer an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Stock Market on June 23, 2017, and warrants to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years. The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Stock Market requirements for an “at the market” offering. Under the terms of the Common Stock Purchase Warrant, the warrants can be settled in unregistered shares. The Warrant Shares qualify for equity classification. The fair value of the allocated proceeds was determined on the relative fair value basis. After deducting for placement agent fees and offering expenses, the aggregate net proceeds from the 2017 PIPE were approximately $47.1 million. On June 27, 2017, in connection with the 2017 PIPE, the Company entered into a registration rights agreement (the “2017 Registration Rights Agreement”) with the Institutional Investors and Dr. Springer. Pursuant to the 2017 Registration Rights Agreement, the Company agreed to prepare and file a registration statement with the SEC within 20 days after the closing of the 2017 PIPE for purposes of registering the resale of the shares of common stock issued and sold in the 2017 PIPE, the Warrant Shares, and any shares of common stock issued as a dividend or other distribution with respect to the shares of common stock or Warrant Shares. The 2017 PIPE registration statement was declared effective by the SEC on July 21, 2017. The Company agreed to indemnify the Institutional Investors and Dr. Springer, their officers, directors, members, employees and agents, successors and assigns under the registration statement from certain liabilities and to pay all fees and expenses (excluding any legal fees of the selling holder(s), and any underwriting discounts and selling commissions) incident to the Company’s obligations under the 2017 Registration Rights Agreement. Warrants During the year ended December 31, 2020, warrant holders exercised 10,233,154 common warrants on a cashless basis and received 5,967,808 shares of common stock. In addition, warrant holders exercised 669,800 common warrants and 8,342,128 pre-funded warrants, and paid the exercise price in cash. On August 31, 2020, in connection with the 2020 Term A Loan, 196,850 equity classified warrants were issued to the Lenders. Number of Warrants Equity Liability classified Total Weighted average Outstanding at December 31, 2019 8,437,747 22,988,501 31,426,248 $ 1.12 Exercises (8,342,128) (10,902,954) (19,245,082) 0.83 Issuance 196,850 — 196,850 2.54 Outstanding at December 31, 2020 292,469 12,085,547 12,378,016 $ 1.60 Common Stock As of December 31, 2020, the Company had 200,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 108,071,249 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics: Voting The common stockholders are entitled to one vote for each share of common stock held with respect to all matters voted on by the stockholders of the Company. Dividends The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through December 31, 2020, no dividends have been declared or paid on common stock. Liquidation Upon liquidation of the Company, the common stockholders are entitled to receive all assets of the Company available for distribution to such stockholders. Reserved Shares The Company has authorized shares of common stock for future issuance as follows: Period ended December 31, 2020 December 31, 2019 Exercise of common warrants 12,378,016 31,426,248 Shares available for future stock incentive awards 4,916,374 1,765,018 Unvested restricted stock units 87,500 181,250 Outstanding common stock options 7,775,249 6,796,669 Total 25,157,139 40,169,185

Stock Incentive Plans

Stock Incentive Plans	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Stock Incentive Plans	Stock Incentive Plans Stock Options The Company maintains the 2008 Stock Incentive Plan (the “2008 Plan”) for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board. At inception of the 2008 Plan, a total of 2,213,412 shares of common stock were authorized for grants under the 2008 Plan. The Company ceased granting awards under the 2008 Plan upon the effectiveness of the 2016 Plan (as defined below); however, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan and the applicable 2008 Plan agreement. Shares subject to awards that were granted under the 2008 Plan and that expire, lapse or terminate following the effectiveness of the 2016 Plan become available under the 2016 Plan as shares available for future grants. All unvested stock options granted under the 2008 Plan may be exercised into restricted stock subject to forfeiture upon termination prior to vesting. On June 7, 2016, the Company’s stockholders approved the 2016 Incentive Award Plan (the “2016 Plan”), which became effective June 21, 2016. Pursuant to the terms of the 2016 Plan, the Board is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. Shares subject to awards that are granted under the 2016 Plan and that expire, lapse or terminate are available for future grants under the 2016 Plan. At inception of the 2016 Plan, a total of 1,210,256 shares of common stock were authorized for future issuance under the 2016 Plan. The number of shares of common stock that may be issued under the 2016 Plan automatically increases on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Board. During the year ended December 31, 2020 and 2019, the number of shares of common stock that may be issued under the 2016 Plan was increased by 3,453,022 shares and 898,871 shares, respectively. As of December 31, 2020, 2,057,593 shares remain available for future issuance under the 2016 Plan. The 2008 Plan and 2016 Plan provide that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2008 Plan and 2016 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date. The Company’s 2018 Employment Inducement Incentive Award Plan (the “2018 Inducement Incentive Award Plan”), which was adopted by the Board on September 25, 2018 without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules (“Rule 5635(c)(4)”), provides for the grant of equity-based awards in the form of non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock or cash based awards. In accordance with Rule 5635(c)(4), awards under the 2018 Inducement Incentive Award Plan may only be made to a newly hired employee who has not previously been a member of the Board, or an employee who is being rehired following a bona fide period of non-employment by the Company, as a material inducement to the employee’s entering into employment with the Company. The Company reserved 1,175,000 shares of its common stock for issuance under the 2018 Inducement Incentive Award Plan. On March 25, 2019, the Board approved the amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of December 31, 2020, there are 1,358,333 shares available for future grant under the 2018 Inducement Incentive Award Plan. The fair value of each option award was estimated on the grant date using the Black-Scholes option pricing model. Expected volatilities are based on the Company’s historical volatility and the historical volatilities of peer companies because the Company's common stock has not traded for a period that is at least equal to the expected term of its stock option awards. The Company uses the “simplified” method to estimate the expected life of options granted and are expected to be outstanding. The risk-free interest rate used is the rate for a U.S. Treasury zero coupon issue with a remaining life consistent with the options expected life on the grant date. The Company has not paid and does not expect to pay in the foreseeable future, any cash dividends. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% based on historical attrition trends. The Company records stock-based compensation expense only on awards that are expected to vest. Employees Effective June 25, 2020, the Company entered into transition agreements with two executive officers, under which the right to exercise their vested options was extended for a period of two years following their respective separation dates. The subsequent stock based compensation amount recognized in connection with the option modification in the second quarter was less than $0.1 million. The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 1.11 % 1.86 % 2.85 % Dividend yield — — — Expected term 6.04 5.94 6.06 Expected volatility 91.00 % 87.66 % 85.17 % Weighted-average fair value of common stock $ 2.49 $ 2.01 $ 8.45 The weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2020, 2019 and 2018 was $1.86, $1.47, and $6.17 respectively. As of December 31, 2020 and December 31, 2019, total unrecognized compensation expense related to unvested employee stock options was $8.0 million and $9.8 million, respectively, which is expected to be recognized over a weighted average period of 2.3 years and 2.5 years, respectively. Non-employee consultants No stock option awards were granted to non-employee consultants during the year ended December 31, 2020. The estimated grant date fair values of non-employee consultants stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2020 2019 2018 Risk-free interest rate — % 1.92 % 2.77 % Dividend yield — — — Expected term 0.00 5.33 5.81 Expected volatility — % 88.60 % 85.86 % The weighted average grant date fair value of stock options granted to non-employee consultants during the years ended December 31, 2019 and 2018 was $1.32 and $6.78, respectively. As of December 31, 2020, no unrecognized compensation expense related to unvested non-employee consultants stock options remained. The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan: Weighted-average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employee awards Outstanding at December 31, 2019 6,323,596 $ 4.91 8.71 $ 1,716 Granted 3,087,709 $ 2.49 Exercised (76,128) $ 2.57 Forfeited (2,033,001) $ 4.67 Outstanding at December 31, 2020 7,302,176 $ 3.98 8.43 $ 4,456 Vested at December 31, 2020 2,684,785 $ 5.72 7.60 $ 1,544 Vested and expected to vest at December 31, 2020 6,830,530 $ 4.08 8.37 $ 4,148 Non-employee awards Outstanding at December 31, 2019 473,073 $ 5.89 6.23 $ 38 Granted — $ — Exercised — $ — Forfeited — $ — Outstanding at December 31, 2020 473,073 $ 5.89 5.23 $ 86 Vested at December 31, 2020 445,855 $ 5.70 5.08 $ 86 Vested and expected to vest at December 31, 2020 473,073 $ 5.89 5.23 $ 86 Restricted Stock Units During the second quarter of 2020, the Company entered into a transition agreement with an executive officer under which previously awarded restricted stock units under the Inducement Incentive Award Plan remain eligible to vest in accordance with their terms, notwithstanding the requirement that the executive remain in continuous service with the Company through the applicable vesting date, if the administrator of the Inducement Incentive Award Plan determines the applicable performance conditions are achieved on or prior to December 31, 2020. In accordance with ASC718, the restricted stock units granted were remeasured with a weighted average fair value of $2.92 per share based on the closing price of the Company’s common stock on the date the transition agreement was executed. Since the performance condition is probable of achievement, the restricted stock units were re-valued, resulting in additional expense of less than $0.1 million. Unrecognized compensation expense for the restricted stock units was $0.5 million as of December 31, 2020, which is expected to be recognized over a weighted average period of 1.9 years. The following table summarizes the status of the Company’s restricted stock units: Number of shares Weighted average Unvested at December 31, 2019 181,250 $ 5.00 Granted — — Vested 93,750 4.07 Forfeited — — Unvested at December 31, 2020 87,500 $ 6.03 Employee Stock Purchase Plan On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 21, 2016. The ESPP is intended to qualify as an “employee stock purchase plan” under Section 423 of the Internal Revenue Code of 1986 with the purpose of providing employees with an opportunity to purchase the Company's common stock through accumulated payroll deductions. Under the ESPP, the Company has set two six-month offering periods during each calendar year, one beginning March 1 st and the other beginning September 1 st of each calendar year, during which employees may elect to have up to 25% of their eligible compensation deducted on each payday on an after-tax basis for use in purchasing the Company's common stock on the last trading day of each offering period, subject to limits imposed by the Internal Revenue Code. The purchase price of the shares may not be less than 85% of the fair market value on the first or last trading day of the offering period, whichever is lower. The first ESPP offering period began on March 1, 2017. At inception of the ESPP, a total of 173,076 shares of common stock were authorized and reserved for future issuance under the ESPP. The number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors. During the years ended December 31, 2020 and 2019, the number of shares of common stock that may be issued under the ESPP was increased by 863,254 shares and 224,717 shares, respectively. During the year ended December 31, 2020, the Company issued 110,212 shares of common stock under the ESPP. As of December 31, 2020, 1,500,448 shares remain available for future issuance under the ESPP. For each of the years ended December 31, 2020 and 2019, the Company recognized $0.1 million of stock-based compensation expense under the ESPP. The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2020 2019 2018 Research and development $ 2,271 $ 2,079 $ 2,453 General and administrative 3,151 3,082 3,261 Total stock-based compensation expense $ 5,422 $ 5,161 $ 5,714

Revenue Arrangements

Revenue Arrangements	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements Swedish Orphan Biovitrum License and Development Agreement On June 11, 2020, the Company and Sobi, entered into the Sobi License. Pursuant to the Sobi License, the Company has agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the Company’s SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037 (the “Compound”) and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, up-front payment of $75 million. Sobi has also agreed to make milestone payments totaling up to $630 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. Pursuant to the Sobi License, the Company has agreed to supply (at cost) quantities of the Compound and ImmTOR as necessary for completion of the two Phase 3 clinical trials of SEL-212 (DISSOLVE I and DISSOLVE II) and a 6-month placebo extension. The Company is required to supply quantities of the Compound until all rights to the Compound and any materials needed to manufacture the Compound are transferred to Sobi. Sobi has agreed to reimburse the Company for all budgeted costs incurred to complete development of SEL-212, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, except for any costs of additional development activities required that are related to ImmTOR and that are unrelated to SEL-212. Sobi will have control and responsibility over all regulatory filings, including any investigational drug applications (IND), biologics license applications (BLA), and marketing authorization applications (MAA) relating to the licensed product. The transactions contemplated by the Sobi License were consummated on July 28, 2020 following the expiration or termination of the required waiting periods under the Hart-Scott-Rodino Antitrust Improvements Act of 1976. Sobi may terminate the Sobi License for any reason upon 180 days’ written notice to the Company, whereby all rights granted under the Sobi License would revert back to the Company. In addition, if Sobi were to terminate the Sobi License, the Company has the option to obtain a license to all patents and know-how necessary to exploit SEL-212 in existence as of the termination date from Sobi in return for making an equitable royalty payment to Sobi. Additionally, on June 11, 2020, the Company entered into the Sobi Purchase Agreement in connection with the Sobi License. The closing of the Sobi Private Placement occurred on July 31, 2020, following the closing of the transactions contemplated under the Sobi License. See Note 10 for details. The Company determined that the Sobi License represents a service arrangement under the scope of ASC 606. In addition, given the Sobi License and Sobi Purchase Agreement were executed contemporaneously and negotiated as a package with a single commercial objective, the Company will account for the two agreements as a single contract. The term of the Sobi License commenced upon the effective date of July 28, 2020 and will continue on a product-by-product basis until the royalty terms for each country have expired. The royalty term for a given product begins upon the first commercial sale of the product in a country and ends at the later of ten years from the first commercial sale, expiration of the last valid patent claim covering the product and expiration of all regulatory exclusivity periods for the product in a country. Given the reversion of the rights under the Sobi License represents a penalty in substance for a termination by Sobi, the contract term would remain the stated term of the Sobi License. The Company determined that the Sobi License contains three distinct performance obligations due to the nature of the promises in the contract, which includes conducting the Phase 3 DISSOLVE trials, Sobi’s option to set-up a second source supplier, and a combined obligation comprised of the delivery of the license to SEL-212, transfer of the know-how and the manufacturing and delivery of SEL-212 supply for development (the “Combined License Obligation”). As the set-up of a second source supplier is optional for Sobi and the Company will be reimbursed at cost for its efforts in the subsequent set-up and technology transfer, the option for this future service was determined to be at a significant and incremental discount to its standalone selling price and treated as a material right in the arrangement, namely a distinct performance obligation. In determining the transaction price, the Company concluded the upfront payment of $75 million and the $5 million development milestone associated with the dosing of the first patient in the Phase 3 DISSOLVE trials will be included in the transaction price. All other development milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of the evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company and probability of success criteria is estimated. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved. In accordance with ASC 606, the Company will only recognize revenue associated with sales-based milestones and royalties when the subsequent sales thresholds are reached and underlying sales occur, respectively. In connection with the Sobi Purchase Agreement, the Company determined that the gross proceeds of $25 million from the Sobi Private Placement included a premium to the fair value of the Company’s shares as of July 28, 2020 equal to approximately $14.5 million. The premium amount will be included in the transaction price for revenue recognition. The Company will estimate and include in the transaction price the total reimbursements to be received from Sobi for both the manufacturing and delivery of the Compound and ImmTOR as well as conducting the Phase 3 DISSOLVE trials. The Company determined that a significant financing component does not exist in its arrangement with Sobi. The Company allocated the transaction price based on the relative standalone selling prices of the three distinct performance obligations. The Company estimated the standalone selling price of conducting the Phase 3 DISSOLVE trials by forecasting its anticipated costs and applying a margin reflective of the industry. The Company must determine the standalone selling price of the second source supplier option by determining the discount given to Sobi multiplied by the likelihood that Sobi will exercise the option in the future. Similar to the Phase 3 program estimate, the Company estimated the discount of the option by forecasting the set-up costs and applying a margin that is reflective of the industry. As the Company will be providing the set-up and technology transfer services and the future supply at cost, the discount of the option is equal to the margin amount. The Company considered discussions with Sobi as well as probability of regulatory success of SEL-212 in determining the likelihood of exercise. The Company estimated the standalone selling price of the Combined License Obligation by utilizing a discounted cash flow model. The Company determined that the delivery of the supply to Sobi best represents the pattern of delivery of the Combined License Obligation as the supply is essential to the utility of the license and know-how. The Company will recognize the revenue allocated to the Combined License Obligation by utilizing the output method. The Company estimated the total supply of the Compound and ImmTOR to be required during the clinical trial period and will recognize revenue as this supply is shipped for use in the clinical trials. The Company will recognize the revenue allocated to the conducting of the Phase 3 DISSOLVE trials obligation by utilizing the input method. The Company estimated the total budgeted costs to be incurred over the Phase 3 DISSOLVE trials and will recognize revenue as these costs are incurred. The Company’s costs best represent the pattern of transfer as these will capture all performance of the trials completed to date and can be readily measured. The Company will recognize the revenue allocated to the second source supplier option when the future services and goods are transferred. As of December 31, 2020, the Company recorded $68.3 million as a short-term contract liability and $24.2 million as a long-term contract liability representing deferred revenue associated with this agreement. In addition, the Company has recorded $1.4 million of contract assets related to incremental costs that would not have been incurred if the Sobi License had not been obtained, of which $1.0 million is presented in prepaid expenses and other current assets and $0.4 million is in other assets in the accompanying Consolidated Balance Sheets. Amortization of contract assets was $0.1 million for the year ended December 31, 2020. As of December 31, 2020, the Company recorded a total outstanding receivable of $6.9 million, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi. Revenue of $16.6 million related to the Sobi License was recognized during the year ended December 31, 2020. Sarepta Therapeutics, Inc. Research License and Option Agreement On June 13, 2020, the Company and Sarepta entered into a Research License and Option Agreement (the “Sarepta Agreement”). Pursuant to the Sarepta Agreement, the Company agreed to grant Sarepta a license under the Company’s intellectual property rights covering the Company’s antigen-specific biodegradable nanoparticle encapsulating ImmTOR to research and evaluate ImmTOR in combination with Sarepta’s adeno-associated virus gene therapy technology, or gene editing technology, using viral or non-viral delivery, to treat Duchenne Muscular Dystrophy and certain Limb-Girdle Muscular Dystrophy subtypes (the “Indications”). Sarepta will have an option term of 24 months during which it can opt-in to obtain an exclusive license to further develop and commercialize the Product to treat at least one Indication, with a potential to extend the option term for an additional fee. The Company will supply ImmTOR to Sarepta for clinical supply on a cost-plus basis. Sarepta paid a $2.0 million up-front payment to the Company upon signing of the Sarepta Agreement, and the Company is eligible to receive additional preclinical payments during the option term. If Sarepta opts-in to an exclusive license agreement, the Company could receive option exercise payments per Indication upon execution of the exclusive license, and the Company would be entitled to significant development and commercial milestone payments and tiered royalties ranging from the mid-to-high single digits based on net sales. Pursuant to the Sarepta Agreement, the Company determined the Sarepta Agreement represents a service arrangement under the scope of ASC 606, with a 24 month contract duration. Given the reversion of the rights under the Sarepta Agreement represents a penalty in substance for a termination by Sarepta, the contract term would remain the stated term of the Sarepta Agreement. The Company determined that the Sarepta Agreement and Supply Obligation including the delivery of the Research License, the Licensed Know-How, the Manufactured Supply and delivery of materials represent a single promise and performance obligation to be transferred to Sarepta over time due to the nature of the promises in the contract. The delivery of the Manufactured Supply is the predominant promise within the arrangement, as it is essential to the utility of the Licensed intellectual property. As such, consideration in the initial transaction price will be allocated to the single performance obligation based on the contractual price. In determining the transaction price, the Company concluded the payment associated with all the performance milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such study milestones is outside the control of the Company and probability of success criteria is estimated. As of December 31, 2020, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved. The Company will recognize the revenue associated with the upfront payment and combined single performance obligation utilizing the output method, over the 24 month term as the Manufactured Supply is delivered to Sarepta. The Company also determined the option to enter into a future commercial license agreement and extend the term of the option does not represent a material right since it was not priced at an incremental discount. Sarepta may terminate the Sarepta Agreement for any reason upon 30 days’ written notice to the Company. The Sarepta Agreement contains other customary terms and conditions, including representations and warranties, covenants, termination, and indemnification obligations in favor of each party. During the year ended December 31, 2020, the Company and Sarepta entered into two amendments relating to an additional feasibility study. Neither of the amendments had a material impact on deferred revenue or revenue recognition. As of December 31, 2020, the Company recorded $2.0 million as a short-term contract liability representing deferred revenue associated with this agreement. Revenue of $0.3 million related to the Sarepta Agreement was recognized during the third quarter and subsequently adjusted during the fourth quarter due to a change in the supply estimate, resulting in de minimis revenue recognition during the year ended December 31, 2020. Asklepios Biopharmaceutical, Inc. License Agreement for Pompe Disease On December 17, 2019, the Company and AskBio entered into a license agreement (the “AskBio License Agreement”). Pursuant to the AskBio License Agreement, AskBio has exercised its option to exclusively license the Company’s intellectual property rights covering the Company’s ImmTOR platform to research, develop, and commercialize certain AAV gene therapy products utilizing ImmTOR, and targeting the GAA gene, or derivatives thereof, to treat Pompe Disease. Pursuant to the AskBio License Agreement and ancillary documents, AskBio agreed to pay to the Company upfront fees of an aggregate of $7.0 million. Assuming successful development and commercialization, the Company could receive up to an additional $237.0 million in development, regulatory, and sales milestone payments. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-to-high single digits. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product until the expiration of the later of (i) ten years after the first commercial sale and (ii) expiration of the last to expire valid claim on patents covering the licensed product. Pursuant to the AskBio License Agreement, the Company will supply AskBio with its ImmTOR platform (“Supply Obligation”) and AskBio will be responsible for all preclinical, clinical and commercial manufacture and supply of licensed products (other than ImmTOR) and carry out all other activities related to the research, development, and commercialization of licensed products at its sole expense, including all regulatory activities related thereto. The Company determined that the AskBio License Agreement and Supply Obligation represent a single promise and performance obligation. This is because AskBio cannot derive benefit from the license without the simultaneous transfer of the patent protected ImmTOR supply. Therefore, the License Obligation and Supply Obligation represent the only promise in the arrangement and are combined as a single performance obligation (the “AskBio License and Supply Obligation”). In determining the transaction price, the Company concluded that the future development milestones, regulatory milestones, sales milestones, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Consideration related to sales-based milestones as well as royalties on net sales upon commercialization by AskBio, will be recognized when the related sales occur, as they were determined to relate predominantly to the intellectual property granted to AskBio and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of December 31, 2020 and December 31, 2019, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. The total initial transaction price of the contract on the effective date was $7.0 million, comprised of a $2.0 million initial up-front payment upon agreement of terms, and a $5.0 million initial up-front execution fee. As of December 31, 2020 and December 31, 2019, the Company recorded $1.7 million as a short-term contract liability and $5.3 million as a long-term contract liability representing deferred revenue associated with this agreement. Revenue will be recognized over the period in which the particles are delivered. No revenue related to the AskBio License Agreement was recognized during the year ended December 31, 2020 as no deliveries were made during the period. Spark Therapeutics, Inc. Spark License Agreement In December 2016, the Company entered into a license and option agreement (“Spark License Agreement”) with Spark pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR platform. The Spark License Agreement provides Spark with certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property, allowing Spark to develop and commercialize gene therapies in combination with ImmTOR for an initial identified target. In addition to an upfront cash payment of $10.0 million under the Spark License Agreement, additional payments of an aggregate of $5.0 million in two payments of $2.5 million each were paid within twelve months of December 2, 2016 (“Contract Date”). The first of the two additional payments was scheduled to be made on or before May 31, 2017 (the “May 2017 License Payment”) (see “Spark Letter Agreement” below) and the second was made on October 31, 2017. Spark may also exercise options to research, develop and commercialize gene therapies utilizing the ImmTOR platform for up to four additional targets. The Company was eligible to receive a variable fee up to $2.0 million for each additional target option elected, dependent on the incidence of the applicable indication. The election period in which Spark could have exercised additional targets under the Spark License Agreement was a term of three years from the Contract Date, which expired on December 1, 2019. Assuming successful development and commercialization, the Company could receive up to an additional $65.0 million in development and regulatory milestone payments and $365.0 million in commercialization milestone payments for each indication. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-single to low-double digits, all of which apply on a target-by-target basis. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product and terminating upon the later of (i) ten years after the first commercial sale, (ii) expiration of the last to expire valid claim on patents covering the jointly invented field specific improvements, or (iii) the expiration of regulatory exclusivity in the applicable country for the licensed product. The Spark License Agreement may be terminated by Spark for convenience upon ninety days’ notice. Either party may terminate the Spark License Agreement on a target-by-target basis for material breach with respect to such target. In December 2016, the Company also entered into a share purchase agreement (the “Spark Purchase Agreement”) with Spark. Pursuant to the Spark Purchase Agreement, the Company sold 197,238 shares of the Company’s common stock to Spark for gross proceeds of $5.0 million, or $25.35 per share of common stock, at an initial closing (the “Initial Closing”). The purchase price per share represents an amount equal to 115% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Contract Date. Beyond the Initial Closing, the Spark Purchase Agreement contemplated potential future sales of shares by the Company to Spark as follows: • First Acquisition Right. During the period beginning on May 1, 2017 and ending on June 1, 2017, Spark had the right (the “First Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million. See “Spark Letter Agreement” below. • Second Acquisition Right. During the period beginning on October 1, 2017 and ending on November 1, 2017, Spark had the right (the “Second Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million. On October 31, 2017 Spark exercised this right and purchased 205,254 shares of common stock from the Company for $5.0 million, or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date. The First Acquisition Rights and Second Acquisition Rights are collectively referred to herein as the “Acquisition Rights”. Under the Spark Purchase Agreement, Spark agreed not to dispose of any of the shares acquired at either the Initial Closing or the from the subsequent Acquisition Rights that it may acquire until January 1, 2018 and, thereafter, transfers are contractually subject to volume limitations applicable to an “affiliate” under Rule 144 of the Securities Act. In connection with the Spark License Agreement and Spark Purchase Agreement, the Company has made contractual payments defined in the MIT license agreement (see Note 14) totaling $2.2 million for the MIT sub-license provided to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The terms of the Spark Purchase Agreement and the Spark License Agreement were negotiated at the same time between the parties and the terms of the Spark Purchase Agreement are referenced in the Spark License Agreement in multiple sections. The pricing and terms of the agreements are unique and must be considered in contemplation with each other. There are provisions within the Spark License Agreement that link to the Spark Purchase Agreement related to provisions that constitute a material breach of the license agreement. Therefore, the Company concluded that the two agreements must be combined and evaluated as a single agreement. While the Spark Purchase Agreement and the Spark License agreement are considered to be a single agreement, the Company determined that the purchase of common stock and future acquisition rights are not within the scope of ASC 606. The Company determined that the initial purchase of common stock combined with the embedded future stock Acquisition Rights had a fair value of $2.7 million and this amount was recorded in equity as of the effective date. The remaining $2.3 million of cash received in exchange for the stock and acquisition rights is included in allocable consideration, as this represents the premium paid by Spark on the purchase of common stock, and should be allocated to the remaining performance obligations. The Company identified the following components of the agreement: (1) certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property and a license to conduct certain research activities under the collaboration, (the “Spark License”), (2) options to research, develop and commercialize gene therapies utilizing the ImmTOR platform for up to four additional target therapy options, (the “Option Obligation”), (3) manufactured supply of ImmTOR, (the “Supply Obligation”) at a discount. In exchange, the Company received an upfront payment of $15.0 million and is eligible to receive additional payments of up to $35.0 million based on the achievement by Spark of future specified development milestones, up to $30.0 million based on the achievement by Spark of future specified regulatory milestones, up to $110.0 million based on the achievement by Spark of future specified commercial milestones, and up to $255.0 million based on the achievement by Spark of future specified sales milestones. The Company will also be eligible to receive tiered royalty payments that reach low double-digits based on future net sales for the duration of the royalty term. The Company determined that the Spark License and Supply Obligation represent a single promise and performance obligation (the “Combined License and Supply Obligation”). This is because Spark cannot derive benefit from the license without the simultaneous transfer of the patent protected ImmTOR supply. The Company also determined that the Option Obligation, which includes the related Supply Obligation, provides the customer with a material right and is considered a performance obligation in the arrangement since it was priced at an incremental discount. Therefore, the Company determined that the Spark agreement contains five distinct performance obligations: the Combined License and Supply Obligation, and the four separate target options. In determining the transaction price, the Company considered the future development milestones, regulatory milestones, commercial milestones, sales milestone, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Separately, any consideration related to sales-based milestones as well as royalties on net sales upon commercialization by Spark, will be recognized when the related sales occur as they were determined to relate predominantly to the intellectual property granted to Spark and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of December 31, 2020, all future milestones are constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. The Company determined that the up-front payment of $12.3 million ($15.0 million, less fair value of the equity totaling $2.7 million as discussed above) was included in the transaction price and was allocated to the performance obligations based on the Company’s best estimate of their relative stand-alone selling prices. The Company allocated $7.1 million to the Combined License and Supply Obligation and $5.2 million to the discount on the target options ($1.3 million for each option) using the relative standalone selling price method to each obligation. The standalone selling price for the Combined License and Supply Obligation was determined using a discounted cash flow model. The standalone selling prices for the target options were determined based on the fair values of the licenses minus the strike prices of the option (the probability of exercise was included in the valuation) as well as the estimated discount of the Supply Obligation. The estimated proceeds expected to be received from the sale of the Supply Obligation were also included in the transaction price for the Combined License and Supply Obligation. The total consideration allocated to the Combined License and Supply Obligation will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term which was initially estimated to be approximately four years. On December 1, 2019, the term for Spark to exercise additional target options expired; the Company recognized $6.7 million in revenue from deferred revenue as originally allocated. In addition, during the year ended December 31, 2019, there were two deliveries resulting in less than $0.1 million of revenue recognized. No revenue related to the Spark License Agreement was recognized during the year ended December 31, 2020 as no deliveries were made during the period. As of December 31, 2020 and December 31, 2019, there was a contract liability of $9.2 million representing deferred revenue presented as long-term associated with this agreement. Spark Letter Agreement On June 6, 2017, the Company and Spark entered into a letter agreement (the “Letter Agreement”), pursuant to which the parties agreed that Spark would make the May 2017 License Payment by June 6, 2017. The May 2017 License Payment was received, and recorded as a liability as of June 30, 2017, of which some or all may potentially constitute the reimbursement described below. The parties also agreed that Spark would be deemed to have delivered notice on May 31, 2017 exercising its right to purchase the shares pursuant to the First Acquisition Right. The Letter Agreement further outlines a cost reimbursement arrangement, pursuant to which the Company agreed to reimburse Spark for all costs and expenses, including the cost of materials provided by the Company, associated with the preclinical research and toxicology studies being performed by Spark for any licensed products for a specified amount of time (the “Reimbursement Period”), in an amount not to exceed $2.5 million. Consistent with the First Acquisition Right, Spark purchased 324,362 shares of common stock pursuant to the Spark Purchase Agreement, as amended by the Letter Agreement, for an aggregate purchase price of $5.0 million, or $15.41 per share of common stock. The purchase price per share represented an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the First Acquisition Right notification date. At the initial contract assessment, the Company allocated $2.7 million to equity (representing the fair value of the initial purchase of common stock combined with the embedded future stock Acquisition Rights). Upon exercise of the First Acquisition Right, the Company recorded the purchase amount to stockholders ’ (deficit) equity. The Company determined that the Letter Agreement resulted in a modification to the original agreement

Related-Party Transactions

Related-Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related-Party Transactions	Related-Party Transactions Consulting Services The Company incurred expenses for consulting services provided by its founders totaling $0.1 million, $0.5 million and $0.2 million during the years ended December 31, 2020, 2019 and 2018, respectively. The Company entered into consulting agreements with its founders to serve on its Scientific Advisory Board, effective January 1, 2020 to December 31, 2021, under which they will be paid quarterly for their services.

Collaboration Agreements

Collaboration Agreements	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration Agreements	Collaboration Agreements Asklepios Biopharmaceutical, Inc. Feasibility Study and License Agreement On August 6, 2019, the Company entered into a feasibility study and license agreement with AskBio (the “AskBio Collaboration Agreement”). Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain AAV gene therapy products utilizing the Company’s ImmTOR platform to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AskBio's AAV gene therapy, or MMA-101, for the treatment of MMA, to mitigate the formation of neutralizing anti-AAV capsid antibodies (the “POC Studies”). If the POC Studies are successful, or the parties otherwise elect to do so, the parties will proceed with a collaboration to pursue the development and commercialization of AAV gene therapy product candidates utilizing ImmTOR for the treatment of certain agreed serious rare and orphan genetic diseases. If the POC Studies fail to demonstrate a proof-of-concept, and the parties do not mutually agree in writing to proceed with the collaboration, the AskBio Collaboration Agreement will expire. The SEL-399 program combines an empty AAV capsid (EMC-101), which is an AAV capsid containing no transgene, with ImmTOR and is being conducted in partnership with AskBio. Building on the preclinical data we have generated showing ImmTOR’s effect on mitigating or reducing the formation of neutralizing antibodies to AAV gene therapies, we have commenced a clinical trial of SEL-399 in healthy adult volunteers in Belgium. The goal of the SEL-399 clinical trial is to demonstrate the appropriate dose of ImmTOR in humans to mitigate the formation of antibodies to AAV capsids used in gene therapies. The Company and AskBio will share responsibility for the research, development and commercialization of products developed under this collaboration. The parties will also share research, development and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby no longer be required to share costs for such products. Each party will receive a percentage of net profits for each product sold under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio Collaboration Agreement, AskBio is responsible for manufacturing the AAV capsids and AAV vectors and the Company is responsible for manufacturing ImmTOR. The AskBio Collaboration Agreement is considered to be within the scope of ASC 808, as both parties are active participants and exposed to the risks and rewards of the collaborative activity. The Company evaluated the terms of the AskBio Collaboration Agreement and have identified the following promises in the arrangement (1) conducting research and development activities to develop and commercialize products under the collaboration, (the “R&D Services”), (2) granting a non-exclusive, non-transferable, royalty-free, fully paid up, worldwide license to certain intellectual property of the Company, (the “IP Rights”) for the purpose of performing the POC Studies, (the “Research License”), (3) granting an exclusive, nontransferable, worldwide license to the IP Rights for use in certain indications (the” Collaboration License”), (4) providing manufactured supply of preclinical and clinical ImmTOR, (the “Manufactured Supply”), (5) participation on identified steering committees responsible for the oversight of the collaboration, (the “JSC Participation”), and (6) granting an exclusive option to obtain a license under the IP Rights to research, develop and commercialize Licensed Products. The Company determined that the R&D Services, Research License, Collaboration License, Manufactured Supply, and JSC Participation were not capable of being distinct, and therefore must be combined into a single performance obligation. Therefore, promises (1) through (5) identified above were combined into a single performance obligation. Furthermore, the Company evaluated the Option Agreement and determined that it does not provide AskBio with a material right under ASC 606 as the option was not priced at a discount (see discussion of the Option exercise in Note 12). The Company noted that AskBio did not meet the definition of a customer within the scope of ASC 606 for any distinct performance obligations as the Company concluded that such items were not an output of the Company’s ordinary activities. As such, the Company determined that the entire arrangement would be accounted for within the scope of ASC 808. In accordance with ASC 808, collaboration expenses are recognized within R&D expense and selling, general and administrative expense on the Company's condensed consolidated statements of operations. For the year ended December 31, 2020, the Company recognized $3.8 million, of collaboration expense under the AskBio Collaboration Agreement in which actual costs incurred by both parties approximate a 50% cost share. Under certain collaborative arrangements, the Company is entitled to reimbursement of certain R&D expense. Activities under collaborative arrangements for which the Company is entitled to reimbursement are considered to be collaborative activities under the scope of ASC 808. For these units of account, the Company does not analogize to ASC 606 or recognize revenue. Rather, the Company analogizes to the guidance in ASC 730, which requires that reimbursements from counterparties be recognized as an offset to the related costs. In accordance with ASC 730, the Company records reimbursement payments received from collaborators as reductions to R&D expense. Massachusetts Institute of Technology On November 25, 2008, the Company entered into an exclusive patent license agreement (the “MIT License”) with the Massachusetts Institute of Technology (“MIT”). The Company received an exclusive royalty-bearing license to utilize patents held by MIT in exchange for upfront consideration and annual license maintenance fees. Such fees are expensed as incurred and have not been material to any period presented. On June 12, 2020, the Company entered into a Fifth Amendment (the “MIT Amendment”) to the MIT License, which is effective as of May 15, 2020. Pursuant to the MIT Amendment, certain of the Company’s diligence obligations were extended, including a diligence obligation to commence a Phase 3 trial for a licensed product by a specific date in the second quarter of 2021. Additionally, certain of the Company’s development and regulatory milestones and payments upon achievement of such milestones were adjusted. As of December 31, 2020, and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the MIT License totaling $2.2 million for the sublicense granted to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The Company made no additional payments during the year ended December 31, 2020. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement (the “3SBio License”) with Shenyang Sunshine Pharmaceutical Co., Ltd. (“3SBio”). The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of December 31, 2020. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company's ImmTOR platform.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe Company provides for income taxes under ASC 740. Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. For the years ended December 31, 2020, 2019 and 2018, the Company did not record a current or deferred income tax expense or benefit. The following table reconciles the federal statutory income rate to the Company's effective income tax rate: Year Ended December 31, 2020 2019 2018 Statutory U.S. federal rate 21.0 % 21.0 % 21.0 % State income taxes - net of federal benefit 5.8 % 6.3 % 7.0 % Permanent items (2.9 %) (2.1 %) (0.5 %) Research tax credits/other 1.0 % 1.1 % 1.6 % Valuation allowance, net (23.6 %) (26.3 %) (29.1 %) Stock Compensation (1.3 %) — % — % Effective income tax rate — % — % — % The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands): Year Ended December 31, 2020 2019 Deferred Tax Assets Net operating loss carryforwards $ 63,678 $ 71,270 Research and development credits 7,632 6,586 Stock-based compensations expense 2,919 2,516 Other expenses 788 812 Deferred revenue 26,699 4,468 Operating lease liability 2,884 102 Patent costs/amortization 5,379 5,111 Gross deferred tax assets 109,979 90,865 Deferred Tax Liabilities Depreciation $ (11) $ (57) Operating lease right-of-use asset (2,991) (82) Gross deferred tax liabilities (3,002) (139) Net deferred tax assets before valuation allowance 106,977 90,726 Valuation allowance (106,977) (90,726) Net deferred tax assets $ — $ — The Company has provided a full valuation allowance against its net deferred tax assets, as the Company believes that it is more likely than not that the deferred tax assets will not be realized. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets and concluded that it is more likely than not that the Company will not realize the benefit of its deferred tax assets. The valuation allowance increased by $16.3 million for the year ended December 31, 2020, primarily as a result of an increase in deferred revenue. The valuation allowance increased by $14.6 million for the year ended December 31, 2019, primarily as a result of an increase in net operating loss. In 2014, the Company's Russian subsidiary was granted a 10 year tax holiday in Russia. The Company's foreign operations continue to benefit from the tax holiday, which is set to expire on December 31, 2023, however the Company is in the process of closing down operations in Russia and does not expect any tax liability. At December 31, 2020, the Company has federal and state net operating loss carryforwards of $234.5 million and $228.5 million, respectively, which will expire at various times beginning 2028. Of the federal net operating losses, $99.6 million can be carried forward indefinitely. The Company has $4.9 million and $3.5 million, respectively, of federal and state research and development tax credit carryforwards, which will expire at various times through 2040. Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 and 383 of the Internal Revenue Code due to ownership change limitations that have occurred previously, or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. As of December 31, 2020, the Company completed a Section 382 study, noting that an ownership change occurred during 2017. The Company has determined that all of the $234.5 million of net operating losses are available in the future, with approximately $36.5 million of that total limited under Section 382 and therefore available for future use through 2028. The Company applies ASC 740, Income Taxes to uncertain tax positions. As of December 31, 2020 and 2019, the Company had no unrecognized tax benefits or related interest and penalties accrued. The Company has not, as of yet, conducted a study of its research and development credit carryforwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statement of operations. As of December 31, 2020, the Company had no accrued interest related to uncertain tax positions. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution PlanThe Company maintains a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. All matching contributions vest ratably over 4 years and participant contributions vest immediately. Contributions by the Company totaled $0.1 million, $0.1 million and $0.2 million during each of the years ended December 31, 2020, 2019 and 2018, respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of December 31, 2020, the Company has an operating lease agreement for an office in Watertown, MA. See Note 8 for additional information regarding the Company's leases. As of December 31, 2020 and December 31, 2019, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. On August 4, 2020, a putative stockholder of Selecta filed a stockholder derivative action, purportedly on behalf of Selecta and against certain current and former members of the Company’s Board of Directors, as well as one affiliated company owned by a current board member, in the Court of Chancery of the State of Delaware, namely Franchi v. Barabe, et al. The complaint alleges that the individual defendants breached their fiduciary duties and committed corporate waste when they authorized a private placement transaction, announced on December 19, 2019, at a price allegedly below fair value. The complaint further alleges that the four defendant directors who participated in the private placement were unjustly enriched in connection with the transaction. On September 25, 2020, the defendants filed a motion to dismiss the lawsuit. On November 6, 2020, the plaintiff filed an amended complaint, and the defendants filed a second motion to dismiss on January 8, 2021. On December 31, 2020, we received a litigation demand letter from two other putative stockholders relating to the same private placement transaction. At this time, the Company has not accrued a liability for this matter, as any liability has been determined to be either not estimable or probable. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non-performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect the Company's business, financial position, results of operations or cash flows.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events “At-the-Market” Offerings Subsequent to December 31, 2020 through March 5, 2021, the Company sold 4,119,613 shares of its common stock pursuant to the 2020 Sales Agreement, at an average price of approximately $4.59 per share for aggregate net proceeds of $18.3 million, after deducting commissions and other transaction costs of $0.6 million which was paid in the three months ended December 31, 2020.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta (RUS), LLC, or Selecta (RUS), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.
Foreign Currency	Foreign CurrencyThe functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ (deficit) equity. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, the valuation of its warrant liabilities and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases.
Cash Equivalents, Investments and Restricted Cash	Cash Equivalents, Investments and Restricted Cash Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these marketable securities and records them at fair value in the accompanying consolidated balance sheets. Investments with less than one year until maturity are classified as short term, while investments with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off-Balance Sheet RiskFinancial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially creditworthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, and common warrants. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2020, given the recent issuance of the Term A Loan under the 2020 Term Loan (each, as defined below), the Company believes the carrying value approximates the fair value of the loan. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three-level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of warrant liabilities were determined using Level 3 inputs.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.
Impairment of Long-Lived Assets	Impairment of Long-Lived AssetsThe Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available, which is at the entity level (“asset group”). An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group.
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the effective interest method and recorded as interest expense.
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non-owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments.
Revenue Recognition	Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC Topic 606, Revenue from Contracts with Customers (ASC 606) , a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Swedish Orphan Biovitrum (“Sobi”), Asklepios Biopharmaceutical, Inc. (“AskBio”), Sarepta Therapeutics, Inc. (“Sarepta”), and Spark Therapeutics, Inc. (“Spark”) (see Note 12) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue: The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Grants and license agreements with customers are accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808 ), and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). The Company early adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which provides guidance on evaluating certain transactions between collaborative arrangement participants. If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC Topic 730, Research and Development (ASC 730) , and records reimbursements from counterparties as an offset to the related costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) up-front fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of research and development (R&D) expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint.
Research and Development Costs	Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, stock-based compensation expenses, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.
Clinical Trial Costs	Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancellable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs. On June 11, 2020, we and Sobi entered into a license and development agreement (the “Sobi License”). Pursuant to the Sobi License, clinical trial costs incurred to complete development of SEL-212, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, will be reimbursed by Sobi. These costs, when reimbursed, will be recognized as revenue consistent with the revenue recognition methodology disclosed in Footnote 12. The reimbursable costs exclude any costs of additional development activities required that are related to ImmTOR and that are unrelated to SEL-212.
Income Taxes	Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.
Warrants	Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity , and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock . Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for all stock-based compensation granted to employees and non-employees using a fair value method. Stock-based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis, net of estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were
Net Loss Per Share	Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares and pre-funded warrants outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and basic and diluted loss per share have been the same.
Contingent Liabilities	Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies
Leases	Leases The Company accounts for its leases in accordance with ASC Topic 842, Leases (ASC 842) , and determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company elected not to recognize leases with a term less than one year on its balance sheet. Operating lease right-of-use (ROU) assets and their corresponding lease liabilities are recorded based on the present value of lease payments over the expected remaining lease term. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. In accordance with the guidance in ASC 842, components of a lease should be split into three categories: lease components (e.g. land, building, etc.), non-lease components (e.g. common area maintenance, consumables, etc.), and non-components (e.g. property taxes, insurance, etc.). Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, the Company elected the practical expedient to not separate lease and non-lease components. The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. Right-of-use assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and estimated incremental borrowing rate upon lease modification. See Note 8 for details.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13) which changes the fair value measurement disclosure requirements of ASC 820. Entities will no longer be required to disclose the amount of, and reasons for, transfers between Level 1 and Level 2 of the fair value hierarchy, the policy of timing of transfers between levels of the fair value hierarchy and the valuation processes for Level 3 fair value measurements. The Company adopted the new standard effective January 1, 2020, and there was no impact on its consolidated financial statements. Not Yet Adopted In October 2020, the FASB issued ASU 2020-10, Codification Improvements , which updates various codification topics by clarifying or improving disclosure requirements to align with the SEC’s regulations. This ASU is effective for public entities for fiscal years beginning after December 15, 2020. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815 – 40) . ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2023, with early adoption permitted no earlier than January 1, 2021. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740) - Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. This ASU is effective for public entities for fiscal years beginning after December 15, 2020. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments . Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses . ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, and reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2022, with early adoption permitted. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows: Year Ended December 31, 2020 2019 2018 Cash and cash equivalents $ 138,685 $ 89,893 $ 37,403 Short-term restricted cash — 279 — Long-term restricted cash 1,379 1,379 279 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 140,064 $ 91,551 $ 37,682
Schedule of Restricted Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows: Year Ended December 31, 2020 2019 2018 Cash and cash equivalents $ 138,685 $ 89,893 $ 37,403 Short-term restricted cash — 279 — Long-term restricted cash 1,379 1,379 279 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 140,064 $ 91,551 $ 37,682

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share	The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per-share data): Year Ended December 31, 2020 2019 2018 Numerator: Net loss attributable to common stockholders $ (68,876) $ (55,350) $ (65,336) Denominator: Weighted-average common shares and pre-funded warrants outstanding—basic and diluted 101,202,176 45,548,511 22,389,286 Net loss per share attributable to common stockholders —basic and diluted $ (0.68) $ (1.22) $ (2.92)
Schedule of Potential Common Shares Issuable Upon Conversion of Warrants	Potential dilutive common share equivalents consist of the following: Year Ended December 31, 2020 2019 2018 Stock options to purchase common stock 7,775,249 6,796,669 4,093,979 Unvested restricted stock units 87,500 181,250 175,000 Stock warrants to purchase common stock 12,378,016 23,084,120 95,619 Total 20,240,765 30,062,039 4,364,598

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Assets Measured at Fair Value on a Recurring Basis	Below is a summary of assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2020 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 80,576 $ 80,576 $ — $ — Total $ 80,576 $ 80,576 $ — $ — Liabilities: Warrant liabilities $ 28,708 $ — $ — $ 28,708 Total $ 28,708 $ — $ — $ 28,708 December 31, 2019 Total (Level 1) (Level 2) (Level 3) Assets: Money market funds $ 50,401 $ 50,401 $ — $ — Total $ 50,401 $ 50,401 $ — $ — Liabilities: Warrant liabilities $ 41,549 $ — $ — $ 41,549 Total $ 41,549 $ — $ — $ 41,549
Schedule of Fair Value Measurement Inputs and Valuation Techniques	A summary of the Black-Scholes pricing model assumptions used to record the fair value of the warrant liability is as follows: December 31, 2020 Risk-free interest rate 0.36 % Dividend yield — Expected life (in years) 3.98 Expected volatility 98.63 %
Schedule Of Changes In The Warrant Liabilities	The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 10), for the year ended December 31, 2020 (in thousands): Warrant liabilities Fair value as of December 31, 2019 $ 41,549 Exercises (23,284) Change in fair value 10,443 Fair value as of December 31, 2020 $ 28,708

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): December 31, December 31, 2020 2019 Laboratory equipment $ 4,427 $ 4,836 Computer equipment and software 532 515 Leasehold improvements 38 278 Furniture and fixtures 327 237 Office equipment 163 135 Construction in process 163 2 Total property and equipment 5,650 6,003 Less accumulated depreciation (4,255) (4,781) Property and equipment, net $ 1,395 $ 1,222

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, December 31, 2020 2019 Payroll and employee related expenses $ 3,049 $ 2,235 Collaboration and licensing 1,350 1,050 Accrued patent fees 534 487 Accrued external research and development costs 2,029 4,379 Accrued professional and consulting services 798 468 Accrued interest 170 82 Issuance costs, December 2019 financing — 4,381 Other 216 410 Accrued expenses $ 8,146 $ 13,492

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Schedule of Lessee, Components Of Operating Lease	As of December 31, 2020 and 2019, the components of the operating leases were as follows (in thousands): December 31, 2020 2019 Assets: Right-of-use asset, net $ 10,948 $ 301 Liabilities: Current operating lease liabilities $ 908 $ 372 Non-current operating lease liabilities 9,647 — Total operating lease liabilities $ 10,555 $ 372
Schedule of Lease, Cost	For the years ended December 31, 2020 and 2019 the components of lease costs were as follows (in thousands): Year Ended December 31, 2020 2019 Operating lease cost $ 2,096 $ 1,365 Variable lease cost 624 828 Short-term lease cost 10 16 Total lease cost $ 2,730 $ 2,209 The following summarizes additional information related to operating leases: December 31, 2020 2019 Weighted-average remaining lease term 7.4 years 0.3 years Weighted-average discount rate 8.9 % 10 %
Schedule of Lessee, Operating Lease, Liability, Maturity	The maturity of the Company's operating lease liabilities as of December 31, 2020 were as follows (in thousands): December 31, 2020 2021 $ 1,812 2022 1,866 2023 1,922 2024 1,980 2025 2,039 Thereafter 4,945 Total future minimum lease payments $ 14,564 Less imputed interest 4,009 Total operating lease liabilities $ 10,555
Schedule of Operating Lease, Lease Income	The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of lease liabilities: $ 2,523 $ 1,482

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule of Future Minimum Payments on the Term Loans	Future minimum principal and interest payments on the 2020 Term Loan as of December 31, 2020 are as follows (in thousands): 2021 2,002 2022 7,343 2023 8,611 2024 8,027 2025 7,274 Total minimum debt payments $ 33,257 Less: Amount representing interest (6,008) Less: Debt discount and deferred charges (2,456) Less: Current portion of loan payable — Loan payable, net of current portion $ 24,793

Equity (Tables)

Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Stockholders' Equity Note, Warrants or Rights	Number of Warrants Equity Liability classified Total Weighted average Outstanding at December 31, 2019 8,437,747 22,988,501 31,426,248 $ 1.12 Exercises (8,342,128) (10,902,954) (19,245,082) 0.83 Issuance 196,850 — 196,850 2.54 Outstanding at December 31, 2020 292,469 12,085,547 12,378,016 $ 1.60
Schedule of Authorized Shares of Common Stock for Future Issuance	The Company has authorized shares of common stock for future issuance as follows: Period ended December 31, 2020 December 31, 2019 Exercise of common warrants 12,378,016 31,426,248 Shares available for future stock incentive awards 4,916,374 1,765,018 Unvested restricted stock units 87,500 181,250 Outstanding common stock options 7,775,249 6,796,669 Total 25,157,139 40,169,185

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 1.11 % 1.86 % 2.85 % Dividend yield — — — Expected term 6.04 5.94 6.06 Expected volatility 91.00 % 87.66 % 85.17 % Weighted-average fair value of common stock $ 2.49 $ 2.01 $ 8.45
Summary of Options Activity	The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan: Weighted-average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employee awards Outstanding at December 31, 2019 6,323,596 $ 4.91 8.71 $ 1,716 Granted 3,087,709 $ 2.49 Exercised (76,128) $ 2.57 Forfeited (2,033,001) $ 4.67 Outstanding at December 31, 2020 7,302,176 $ 3.98 8.43 $ 4,456 Vested at December 31, 2020 2,684,785 $ 5.72 7.60 $ 1,544 Vested and expected to vest at December 31, 2020 6,830,530 $ 4.08 8.37 $ 4,148 Non-employee awards Outstanding at December 31, 2019 473,073 $ 5.89 6.23 $ 38 Granted — $ — Exercised — $ — Forfeited — $ — Outstanding at December 31, 2020 473,073 $ 5.89 5.23 $ 86 Vested at December 31, 2020 445,855 $ 5.70 5.08 $ 86 Vested and expected to vest at December 31, 2020 473,073 $ 5.89 5.23 $ 86
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	The following table summarizes the status of the Company’s restricted stock units: Number of shares Weighted average Unvested at December 31, 2019 181,250 $ 5.00 Granted — — Vested 93,750 4.07 Forfeited — — Unvested at December 31, 2020 87,500 $ 6.03
Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock	The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2020 2019 2018 Research and development $ 2,271 $ 2,079 $ 2,453 General and administrative 3,151 3,082 3,261 Total stock-based compensation expense $ 5,422 $ 5,161 $ 5,714
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of non-employee consultants stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2020 2019 2018 Risk-free interest rate — % 1.92 % 2.77 % Dividend yield — — — Expected term 0.00 5.33 5.81 Expected volatility — % 88.60 % 85.86 %

Revenue Arrangements (Tables)

Revenue Arrangements (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Schedule of Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2020 (in thousands): Balance at Balance at beginning of period Additions Deductions end of period Twelve Months Ended December 31, 2020 Contract liabilities: Deferred revenue $ 16,354 $ 104,158 $ (9,716) $ 110,796 Total contract liabilities $ 16,354 $ 104,158 $ (9,716) $ 110,796

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	For the years ended December 31, 2020, 2019 and 2018, the Company did not record a current or deferred income tax expense or benefit. The following table reconciles the federal statutory income rate to the Company's effective income tax rate: Year Ended December 31, 2020 2019 2018 Statutory U.S. federal rate 21.0 % 21.0 % 21.0 % State income taxes - net of federal benefit 5.8 % 6.3 % 7.0 % Permanent items (2.9 %) (2.1 %) (0.5 %) Research tax credits/other 1.0 % 1.1 % 1.6 % Valuation allowance, net (23.6 %) (26.3 %) (29.1 %) Stock Compensation (1.3 %) — % — % Effective income tax rate — % — % — %
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands): Year Ended December 31, 2020 2019 Deferred Tax Assets Net operating loss carryforwards $ 63,678 $ 71,270 Research and development credits 7,632 6,586 Stock-based compensations expense 2,919 2,516 Other expenses 788 812 Deferred revenue 26,699 4,468 Operating lease liability 2,884 102 Patent costs/amortization 5,379 5,111 Gross deferred tax assets 109,979 90,865 Deferred Tax Liabilities Depreciation $ (11) $ (57) Operating lease right-of-use asset (2,991) (82) Gross deferred tax liabilities (3,002) (139) Net deferred tax assets before valuation allowance 106,977 90,726 Valuation allowance (106,977) (90,726) Net deferred tax assets $ — $ —

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	$ 140,064	$ 91,551	$ 37,682	$ 71,027
Restricted cash and cash equivalents	1,400
Accumulated deficit	404,629	$ 335,753
Russian subsidiary
Cash and Cash Equivalents [Line Items]
Cash maintained in Russian bank accounts	$ 300

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020USD ($)segment
Accounting Policies [Abstract]
Number of operating segments | segment	1
Loss on sale of investments	$ 0
Other than temporary impairment losses on investments	$ 0

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 138,685	$ 89,893	$ 37,403
Short-term restricted cash	0	279	0
Long-term restricted cash	1,379	1,379	279
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 140,064	$ 91,551	$ 37,682	$ 71,027

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) $ in Millions	Dec. 31, 2020USD ($)
Russian subsidiary
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 0.3

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Laboratory equipment, software and office equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Impairment of Long‑Lived Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Impairment of intangible assets, finite-lived	$ 0	$ 0

Net Loss Per Share - Narrative

Net Loss Per Share - Narrative (Details)	3 Months Ended
	Jun. 30, 2020shares
Exercise of common warrants | December 2019 Financing
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Aggregate shares of common stock at a purchase price (in shares)	8,342,128

Net Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss attributable to common stockholders	$ (68,876)	$ (55,350)	$ (65,336)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)	101,202,176	45,548,511	22,389,286
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.68)	$ (1.22)	$ (2.92)

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Potential common shares
Total (in shares)	20,240,765	30,062,039	4,364,598
Stock options to purchase common stock
Potential common shares
Total (in shares)	7,775,249	6,796,669	4,093,979
Unvested restricted stock units
Potential common shares
Total (in shares)	87,500	181,250	175,000
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	12,378,016	23,084,120	95,619

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Liabilities:
Warrant liabilities	$ 28,708	$ 41,549
Recurring
Assets:
Total	80,576	50,401
Liabilities:
Warrant liabilities	28,708	41,549
Total	28,708	41,549
Recurring | (Level 1)
Assets:
Total	80,576	50,401
Liabilities:
Warrant liabilities	0	0
Total	0	0
Recurring | (Level 2)
Assets:
Total	0	0
Liabilities:
Warrant liabilities	0	0
Total	0	0
Recurring | (Level 3)
Assets:
Total	0	0
Liabilities:
Warrant liabilities	28,708	41,549
Total	28,708	41,549
Recurring | Money market funds
Assets:
Money market funds	80,576	50,401
Recurring | Money market funds | (Level 1)
Assets:
Money market funds	80,576	50,401
Recurring | Money market funds | (Level 2)
Assets:
Money market funds	0	0
Recurring | Money market funds | (Level 3)
Assets:
Money market funds	$ 0	$ 0

Fair Value Measurements - Assum

Fair Value Measurements - Assumptions used to record the fair value of the warrants (Details) - Valuation Technique, Option Pricing Model	Dec. 31, 2020
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0036
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0
Expected life (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	3 years 11 months 23 days
Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.9863

Fair Value Measurements - Chang

Fair Value Measurements - Change in the warrant liabilities (Details) - Warrant liabilities $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value as of December 31, 2019	$ 41,549
Exercises	(23,284)
Change in fair value	10,443
Fair value as of December 31, 2020	$ 28,708

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 5,650	$ 6,003
Less accumulated depreciation	(4,255)	(4,781)
Property and equipment, net	1,395	1,222
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	4,427	4,836
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	532	515
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	38	278
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	327	237
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	163	135
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 163	$ 2

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation expense		$ 0.6	$ 0.7	$ 1
Property, Plant and Equipment [Line Items]
Accelerated depreciation expense (less than)		$ 0.1
Construction in process
Property, Plant and Equipment [Line Items]
Capitalized website development costs	$ 0.1

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 3,049	$ 2,235
Collaboration and licensing	1,350	1,050
Accrued patent fees	534	487
Accrued external research and development costs	2,029	4,379
Accrued professional and consulting services	798	468
Accrued interest	170	82
Issuance costs, December 2019 financing	0	4,381
Other	216	410
Accrued expenses	$ 8,146	$ 13,492

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2019USD ($)ft²	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Mar. 31, 2020USD ($)extension	Mar. 31, 2019USD ($)	Jan. 01, 2019USD ($)	Oct. 31, 2017ft²	Dec. 31, 2012USD ($)
Operating Leased Assets [Line Items]
Right of use asset, net		$ 10,948	$ 301
Lease liabilities		10,555	372
Operating lease, remaining lease term							15 months
Discount rate							10.00%
Number of extension on lease term | extension					1
Lessee-paid construction costs		$ 2,730	$ 2,209
Operating lease, weighted average discount rate, percent		8.90%	10.00%
Operating lease cost		$ 2,096	$ 1,365
Laboratory and office space
Operating Leased Assets [Line Items]
Leasehold improvements gross (up to)									$ 700
75 North Beacon, Watertown, MA
Operating Leased Assets [Line Items]
Right of use asset, net						$ 0	$ 200
Area of additional office space leased | ft²								5,100
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Right of use asset, net					$ 11,800
Lease liabilities					$ 11,800
Term of contract	8 years
Area of additional office space leased | ft²	25,078
Lessee-paid construction costs	$ 800
Operating lease, expense	200
Moscow, Russia Lease
Operating Leased Assets [Line Items]
Operating lease, expense				$ 2,000
Operating lease cost		2,700	$ 2,100
Letter of credit
Operating Leased Assets [Line Items]
Restricted cash	$ 1,400	$ 300
Minimum
Operating Leased Assets [Line Items]
Term of contract		2 years
Operating lease, renewal term		1 year
Maximum
Operating Leased Assets [Line Items]
Term of contract		8 years
Operating lease, renewal term		5 years
Cumulative Effect, Period of Adoption, Adjustment
Operating Leased Assets [Line Items]
Right of use asset, net				1,600
Lease liabilities				1,800
Deferred rent and incentives reversal				$ 200

Leases - Components of Operatin

Leases - Components of Operating Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Right-of-use asset, net	$ 10,948	$ 301
Liabilities:
Current operating lease liabilities	908	372
Non-current operating lease liabilities	9,647	0
Total operating lease liabilities	$ 10,555	$ 372

Leases - Components of lease co

Leases - Components of lease costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 2,096	$ 1,365
Variable lease cost	624	828
Short-term lease cost	10	16
Total lease cost	$ 2,730	$ 2,209

Leases - Operating Lease, Liabi

Leases - Operating Lease, Liability, Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2021	$ 1,812
2022	1,866
2023	1,922
2024	1,980
2025	2,039
Thereafter	4,945
Total future minimum lease payments	14,564
Less imputed interest	4,009
Lease liabilities	$ 10,555	$ 372

Leases - Other Information (Det

Leases - Other Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities:	$ 2,523	$ 1,482

Leases - Lease Term and Discoun

Leases - Lease Term and Discount Rate (Details)	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease, weighted average remaining lease term	7 years 4 months 24 days	3 months 18 days
Operating lease, weighted average discount rate, percent	8.90%	10.00%

Debt - Term Loans (Narrative) (

Debt - Term Loans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	Aug. 31, 2020	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 12, 2017	Jun. 27, 2017
Line of Credit Facility [Line Items]
Final payment fee			$ 2,300
Class of warrant or right, number of securities called by warrants or rights (in shares)	196,850
Exercise price (in dollars per share)	$ 2.54		$ 1.60	$ 1.12
Class of warrant or right, expiration period	10 years
Issuance of common warrants with long-term debt, net	$ 400		$ 444
Loss on extinguishment of debt			461	$ 0	$ 0
Interest expense			$ 1,556	1,519	1,494
Stock warrants to purchase common stock
Line of Credit Facility [Line Items]
Exercise price (in dollars per share)							$ 17.71
2020 Term Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	35,000
Debt instrument, additional accrued interest in the event of default			0.050
Debt instrument, redemption price, percentage of principal amount redeemed			9.00%
Debt instrument, default, bankruptcy, or insolvency value of triggering accelerated redemption	500
Debt issuance costs, line of credit arrangements (less than)	400
Term loan facility			$ 25,000
2020 Term Loans | Maximum
Line of Credit Facility [Line Items]
Debt instrument, prepayment fee, percentage			0.030
2020 Term Loans | Minimum
Line of Credit Facility [Line Items]
Debt instrument, prepayment fee, percentage			0.010
2020 Term Loans | Debt Instrument, Redemption, Period One
Line of Credit Facility [Line Items]
Basis spread on variable rate			7.90%
2020 Term Loans | Debt Instrument, Redemption, Period Two
Line of Credit Facility [Line Items]
Basis spread on variable rate			4.65%
2020 Term Loans | Debt Instrument, Redemption, Period Three
Line of Credit Facility [Line Items]
Basis spread on variable rate			10.00%
Term A Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	25,000
Term B Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	10,000
Term B Loans | Stock warrants to purchase common stock
Line of Credit Facility [Line Items]
Sale common stock	200,000
2017 Term Loans
Line of Credit Facility [Line Items]
Debt issuance costs, line of credit arrangements (less than)						$ 100
Repayments of lines of credit	$ 13,700
Loss on extinguishment of debt		$ 500
Term loan facility				18,200		$ 21,000
Interest expense			$ 1,600	$ 1,500	$ 1,500
2017 Term Loans | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate			0.50%

Debt - Schedule of Future Minim

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Dec. 31, 2020USD ($)
Year ended December 31,
2021	$ 2,002
2022	7,343
2023	8,611
2024	8,027
2025	7,274
Total minimum debt payments	33,257
Less: Amount representing interest	(6,008)
Less: Debt discount and deferred charges	(2,456)
Less: Current portion of loan payable	0
Loan payable, net of current portion	$ 24,793

Equity - Narrative (Details)

Equity - Narrative (Details)	Aug. 31, 2020$ / sharesshares	Aug. 06, 2020USD ($)	Jul. 28, 2020USD ($)	Jun. 11, 2020USD ($)$ / sharesshares	Dec. 18, 2019USD ($)$ / sharesshares	Jun. 27, 2017USD ($)$ / sharesshares	Aug. 31, 2017USD ($)	Dec. 31, 2020USD ($)vote$ / sharesshares	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)vote$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017shares
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, outstanding (in shares)								12,378,016,000		12,378,016,000	31,426,248,000
Exercise price (in dollars per share) | $ / shares	$ 2.54							$ 1.60		$ 1.60	$ 1.12
Warrant liabilities | $								$ 28,708,000		$ 28,708,000	$ 41,549,000
Issuance of common stock upon exercise of common warrants (in shares)										19,245,082,000
Stock issued during period, warrant issued (in shares)										196,850,000
Common stock, shares authorized (in shares)								200,000,000		200,000,000	200,000,000
Common stock, par value (in dollars per share) | $ / shares								$ 0.0001		$ 0.0001	$ 0.0001
Common stock, shares issued								108,071,249		108,071,249	86,325,547
Common stock, shares outstanding								108,071,249		108,071,249	86,325,547
Number of votes per share that common stockholders are entitled to | vote								1		1
Dividends declared or paid on common stock | $										$ 0
August 2020 Shelf Registration Statement
Subsidiary, Sale of Stock [Line Items]
Sale common stock | $		$ 200,000,000
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale common stock | $							$ 50,000,000
Sale of stock, number of shares issued in transaction (in shares)										1,069,486	615,453
Share price (in dollars per share) | $ / shares								$ 2.16		$ 2.16	$ 1.84
Shares issued, value | $										$ 2,108,000	$ 1,006,000	$ 0
2020 Sale Agreement - At-the-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale common stock | $							$ 50,000,000
Issuance costs | $								$ 600,000
SOBI Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Sale common stock | $				$ 25,000,000
Sale of stock, number of shares issued in transaction (in shares)				5,416,390
Share price (in dollars per share) | $ / shares				$ 4.6156
Sale of stock, percentage of ten-day volume weighted average price of common stock				120.00%
Lock-up from closing, term										1 year
Redemption premium | $			$ 14,500,000
December 2019 Financing
Subsidiary, Sale of Stock [Line Items]
Sale common stock | $					$ 65,600,000
Sale of stock, number of shares issued in transaction (in shares)					37,634,883
Share price (in dollars per share) | $ / shares					$ 1.46
Class of warrant or right, outstanding (in shares)					22,988,501
Warrant Issued | $					$ 40,700,000
Issuance costs | $									$ 4,400,000
Warrant liabilities | $								$ 28,700,000		$ 28,700,000
Warrant, increase (decrease) in equity, amount | $										$ (10,400,000)	$ (900,000)
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)						2,750,000
Share price (in dollars per share) | $ / shares						$ 16
Shares issued, value | $						$ 47,100,000
Common stock, shares authorized (in shares)								200,000,000		200,000,000
Common stock, par value (in dollars per share) | $ / shares								$ 0.0001		$ 0.0001
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares						$ 17.71
Springer Purchase Agreement | Common stock
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares)						338,791
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)						79,130
Exercise price (in dollars per share) | $ / shares						$ 17.71
Warrants and rights outstanding, term						5 years
Share price (in dollars per share) | $ / shares						$ 0.125
Pre-Funded Warrant | December 2019 Financing
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, number of warrants exercised (in shares)					8,342,128
Exercise price (in dollars per share) | $ / shares					$ 0.0001
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, outstanding (in shares)								292,469,000		292,469,000	8,437,747,000
Class of warrant or right, number of warrants exercised (in shares)										10,233,154
Issuance of common stock upon exercise of common warrants (in shares)										8,342,128,000
Stock issued during period, warrant issued (in shares)	196,850									196,850,000
Common stock
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares)											22,188,706
Issuance of common stock upon exercise of common warrants (in shares)										5,967,808
Common stock, shares outstanding								108,071,249		108,071,249	86,325,547	22,471,776	22,343,254
Common Warrant | December 2019 Financing
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares					0.125
Class of warrant or right, number of warrants exercised (in shares)										669,800
Exercise price (in dollars per share) | $ / shares					$ 1.46

Equity - Warranty Activity (Det

Equity - Warranty Activity (Details) - $ / shares	Aug. 31, 2020	Dec. 31, 2020
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)		31,426,248,000
Exercises (in shares)		(19,245,082,000)
Issuances (in shares)		196,850,000
Class of warrant or right, outstanding, ending balance (in shares)		12,378,016,000
Class Of Warrant Or Right Weighted Average Exercise Price [Roll Forward]
Exercise price, beginning balance (in dollars per share)		$ 1.12
Exercises (in dollars per share)		0.83
Issuance (in dollars per share)		2.54
Exercise price, ending balance (in dollars per share)	$ 2.54	$ 1.60
Exercise of common warrants
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)		8,437,747,000
Exercises (in shares)		(8,342,128,000)
Issuances (in shares)	196,850	196,850,000
Class of warrant or right, outstanding, ending balance (in shares)		292,469,000
Exercise of common warrants
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)		22,988,501,000
Exercises (in shares)		(10,902,954,000)
Issuances (in shares)		0
Class of warrant or right, outstanding, ending balance (in shares)		12,085,547,000

Equity - Schedule of Authorized

Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Total (in shares)	25,157,139	40,169,185
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	4,916,374	1,765,018
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	87,500	181,250
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	7,775,249	6,796,669
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	12,378,016	31,426,248

Stock Incentive Plans - Narrati

Stock Incentive Plans - Narrative (Details)	Jun. 25, 2020	Jun. 21, 2016shares	Dec. 31, 2020USD ($)$ / sharesshares	Jun. 30, 2020USD ($)	Dec. 31, 2020USD ($)$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / shares	Mar. 25, 2019shares	Sep. 25, 2018shares	Dec. 31, 2008shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			25,157,139		25,157,139	40,169,185
Award vesting term	2 years
Stock‑based compensation expense | $				$ 100,000	$ 5,422,000	$ 5,161,000	$ 5,714,000
Unrecognized compensation expense related to unvested non-employee stock options | $			$ 0		$ 0
Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			4,916,374		4,916,374	1,765,018
Estimated forfeitures rate					10.00%
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares			$ 1.86		$ 1.47		$ 6.17
Unrecognized compensation expense related to unvested employee stock options | $			$ 8,000,000		$ 8,000,000	$ 9,800,000
Weighted average period for recognition					2 years 3 months 18 days	2 years 6 months
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares						$ 1.32	$ 6.78
Granted (in shares)					0
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			87,500		87,500	181,250
Grants in period (in dollars per share) | $ / shares					$ 0
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)										2,213,412
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		1,210,256
Automatic increase in the number of shares that may be issued (as a percent)		0.04
Number of shares authorized, increase (in shares)					3,453,022	898,871
Common stock authorized and reserved for future issuance (in shares)			2,057,593		2,057,593
2016 Plan | Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants, percentage (less than)					10.00%
Award vesting term					4 years
Award expiration term					10 years
2016 Plan | Employee stock option grants | Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)					100.00%
2016 Plan | Employee stock option grants | Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)					110.00%
Combined voting power of participants, percentage (less than)					10.00%
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)			1,358,333		1,358,333
Common stock authorized and reserved for future issuance (in shares)								2,000,000	1,175,000
Employment Inducement Incentive Award Plan | Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for recognition					1 year 10 months 24 days
Unrecognized compensation expense | $			$ 500,000		$ 500,000
Employment Inducement Incentive Award Plan | Unvested restricted stock units | Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in dollars per share) | $ / shares			$ 2.92
Vested in period | $			$ 100,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Automatic increase in the number of shares that may be issued (as a percent)		0.01
Number of shares authorized, increase (in shares)					863,254	224,717
Common stock authorized and reserved for future issuance (in shares)		173,076	1,500,448		1,500,448
Exercise price of incentive stock options (not less than)		85.00%
Stock‑based compensation expense | $					$ 100,000	$ 100,000

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.11%	1.86%	2.85%
Dividend yield	0.00%	0.00%	0.00%
Expected term	6 years 14 days	5 years 11 months 8 days	6 years 21 days
Expected volatility	91.00%	87.66%	85.17%
Weighted-average fair value of common stock (in dollars per share)	$ 2.49	$ 2.01	$ 8.45
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.00%	1.92%	2.77%
Dividend yield	0.00%	0.00%	0.00%
Expected term	0 years	5 years 3 months 29 days	5 years 9 months 21 days
Expected volatility	0.00%	88.60%	85.86%

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Non-employee awards
Number of Options
Granted (in shares)	0
2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan | Employee awards
Number of Options
Beginning balance (in shares)	6,323,596
Granted (in shares)	3,087,709
Exercised (in shares)	(76,128)
Forfeited (in shares)	(2,033,001)
Ending balance (in shares)	7,302,176	6,323,596
Vested (in shares)	2,684,785
Vested and expected to vest (in shares)	6,830,530
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 4.91
Granted (in dollars per share)	2.49
Exercised (in dollars per share)	2.57
Forfeited (in dollars per share)	4.67
Ending balance (in dollars per share)	3.98	$ 4.91
Vested (in dollars per share)	5.72
Vested and expected to vest (in dollars per share)	$ 4.08
Weighted-average remaining contractual term
Outstanding, term	8 years 5 months 4 days	8 years 8 months 15 days
Vested, term	7 years 7 months 6 days
Vested and expected to vest, term	8 years 4 months 13 days
Aggregate intrinsic value
Outstanding	$ 4,456	$ 1,716
Vested	1,544
Vested and expected to vest	$ 4,148
2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan | Non-employee awards
Number of Options
Beginning balance (in shares)	473,073
Granted (in shares)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Ending balance (in shares)	473,073	473,073
Vested (in shares)	445,855
Vested and expected to vest (in shares)	473,073
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 5.89
Granted (in dollars per share)	0
Exercised (in dollars per share)	0
Forfeited (in dollars per share)	0
Ending balance (in dollars per share)	5.89	$ 5.89
Vested (in dollars per share)	5.70
Vested and expected to vest (in dollars per share)	$ 5.89
Weighted-average remaining contractual term
Outstanding, term	5 years 2 months 23 days	6 years 2 months 23 days
Vested, term	5 years 29 days
Vested and expected to vest, term	5 years 2 months 23 days
Aggregate intrinsic value
Outstanding	$ 86	$ 38
Vested	86
Vested and expected to vest	$ 86

Stock Incentive Plans - Restric

Stock Incentive Plans - Restricted Stock Units (Details) - Unvested restricted stock units	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested at beginning of period (in shares) | shares	181,250
Grants in period (in shares) | shares	0
Vested in period (in shares) | shares	93,750
Forfeited in period (in shares) | shares	0
Unvested at end of period (in shares) | shares	87,500
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unvested at beginning of period (in dollars per share) | $ / shares	$ 5
Grants in period (in dollars per share) | $ / shares	0
Vested in period (in dollars per share) | $ / shares	4.07
Forfeited in period (in dollars per share) | $ / shares	0
Unvested at end of period (in dollars per share) | $ / shares	$ 6.03

Stock Incentive Plans - Employe

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) $ in Thousands	Jun. 21, 2016shares	Jun. 30, 2020USD ($)	Dec. 31, 2020USD ($)offeringPeriodshares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			25,157,139	40,169,185
Stock-based compensation expense | $		$ 100	$ 5,422	$ 5,161	$ 5,714
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of six-month offering periods under the ESPP | offeringPeriod			2
Eligible compensation withheld for use (up to) (as a percent)	25.00%
Purchase price of shares (not less than) (as a percent)	85.00%
Common stock authorized and reserved for future issuance (in shares)	173,076		1,500,448
Number of shares of common stock outstanding (as a percent)	0.01
Number of shares authorized, increase (in shares)			863,254	224,717
Shares of common stock issued to employees under the ESPP (in shares)			110,212
Stock-based compensation expense | $			$ 100	$ 100

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 100	$ 5,422	$ 5,161	$ 5,714
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense		2,271	2,079	2,453
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense		$ 3,151	$ 3,082	$ 3,261

Revenue Arrangements - Narrativ

Revenue Arrangements - Narrative (Details)	Jul. 28, 2020USD ($)	Jun. 13, 2020USD ($)	Jun. 11, 2020USD ($)$ / sharesshares	Dec. 17, 2019USD ($)	Jun. 05, 2019USD ($)	Oct. 31, 2017USD ($)$ / sharesshares	Jun. 06, 2017USD ($)$ / sharesshares	Dec. 02, 2016USD ($)obligationtargetcontract	Dec. 31, 2016USD ($)$ / sharesshares	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)agreementobligation	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2020USD ($)agreementobligation	Dec. 01, 2019USD ($)	Dec. 31, 2017USD ($)
Disaggregation of Revenue [Line Items]
Short-term contract liability											$ 72,050,000	$ 1,674,000		$ 72,050,000
Long-term contract liability											38,746,000	14,680,000		38,746,000
Contract assets amortization											100,000
Accounts receivable											7,224,000	5,000,000		7,224,000
Revenue recognized											9,716,000
Number of additional targets | target								4
Remaining performance obligation											110,800,000			110,800,000
Deferred revenue related to agreement											110,796,000	16,354,000		110,796,000
Grant and collaboration revenue											16,597,000	6,677,000	$ 903,000
Retained earnings (accumulated deficit)											$ (404,629,000)	(335,753,000)		(404,629,000)
Contract liability, period of recognition											3 years
SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Sale common stock			$ 25,000,000
Redemption premium	$ 14,500,000
Short-term contract liability											$ 68,300,000			68,300,000
Long-term contract liability											24,200,000			24,200,000
Contract assets											1,400,000			1,400,000
Accounts receivable											6,900,000			6,900,000
Revenue recognized											16,600,000
Sale of stock, number of shares issued in transaction (in shares) | shares			5,416,390
Share price (in dollars per share) | $ / shares			$ 4.6156
SOBI Purchase Agreement | Prepaid Expenses and Other Current Assets
Disaggregation of Revenue [Line Items]
Contract assets											1,000,000			1,000,000
SOBI Purchase Agreement | Other Current Assets
Disaggregation of Revenue [Line Items]
Contract assets											$ 400,000			$ 400,000
Collaborative Arrangement
Disaggregation of Revenue [Line Items]
Upfront cash payment			$ 75,000,000
Sales milestone payments			630,000,000
Period after first commercial sale when the Company is eligible to receive royalties											10 years
Number of obligations | obligation											3			3
Development milestone			$ 5,000,000
Spark Letter Agreement
Disaggregation of Revenue [Line Items]
Sale common stock						$ 2,500,000
Premium of average daily VWAP (as a percent)						115.00%
Aggregate purchase price							$ 5,000,000
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement (in shares) | shares						205,254	324,362
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights								$ 2,700,000
Deferred revenue related to agreement						$ 2,500,000
Aggregate purchase price per share (in dollar per share) | $ / shares						$ 24.36	$ 15.41
Reimbursement invoices					$ 1,200,000
Spark License Agreement
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement															$ 6,700,000
Sarepta Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Upfront cash payment		$ 2,000,000
Short-term contract liability											$ 2,000,000			$ 2,000,000
Revenue recognized										$ 300,000
Option term											24 months
License Agreement For Pompe Disease
Disaggregation of Revenue [Line Items]
Upfront cash payment				$ 7,000,000
Sales milestone payments				$ 237,000,000
Period after first commercial sale when the Company is eligible to receive royalties				10 years
Short-term contract liability											$ 1,700,000	1,700,000		1,700,000
Long-term contract liability											5,300,000	5,300,000		$ 5,300,000
Revenue recognized											$ 0
Initial up-front cash payment				$ 2,000,000
Initial upfront execution fee payment				$ 5,000,000
Spark License Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment								10,000,000
Sales milestone payments								$ 255,000,000
Period after first commercial sale when the Company is eligible to receive royalties								10 years
Sale common stock									$ 5,000,000
Aggregate additional payments due within twelve months								$ 5,000,000
Term loan number of payments | contract								2
Two payments due within 12 months of contract date								$ 2,500,000
Number of additional targets | target								4
Variable fee the Company is eligible to receive for each additional target option elected								$ 2,000,000
Election period to exercise additional targets								3 years
Additional development and regulatory milestone payments								$ 65,000,000
Commercialization milestone payments								$ 365,000,000
Period for prior written notice to terminate license								90 days
Sale of stock, number of shares issued in transaction (in shares) | shares									197,238
Share price (in dollars per share) | $ / shares									$ 25.35
Premium of average daily VWAP (as a percent)									115.00%
Number of agreements combined and evaluated into a single agreement | agreement											2			2
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights								$ 2,700,000
Remaining performance obligation								12,300,000			$ 2,300,000			$ 2,300,000
Performance obligation per each option								1,300,000
Deferred revenue related to agreement											9,200,000	9,200,000		9,200,000
Spark License Agreement | MIT
Disaggregation of Revenue [Line Items]
License and option agreement, additional sub-license payment received liability											2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement											400,000
Spark License Agreement | Consideration for Promises
Disaggregation of Revenue [Line Items]
Upfront cash payment								15,000,000
Spark License Agreement | Development Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments								35,000,000
Spark License Agreement | Regulatory Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments								30,000,000
Spark License Agreement | Commercial Milestones
Disaggregation of Revenue [Line Items]
Commercialization milestone payments								$ 110,000,000
Spark License Agreement | License and Supply Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation								5
Remaining performance obligation								$ 7,100,000
Spark License Agreement | Discount on Option Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation								4
Remaining performance obligation								$ 5,200,000
First Acquisition Right
Disaggregation of Revenue [Line Items]
Aggregate purchase price						$ 5,000,000
Second Acquisition Right
Disaggregation of Revenue [Line Items]
Share price (in dollars per share) | $ / shares						$ 24.36
Premium of average daily VWAP (as a percent)						115.00%
Aggregate price of shares Spark will have the right to purchase						$ 5,000,000
Skolkovo
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement											100,000			100,000		$ 200,000
Payments received														$ 2,000,000
Grants from entities, unused grant funds													100,000
Overspent costs													$ 100,000
Skolkovo | Cumulative Effect, Period of Adoption, Adjustment
Disaggregation of Revenue [Line Items]
Retained earnings (accumulated deficit)																$ 1,800,000
Grant Revenue
Disaggregation of Revenue [Line Items]
Grant and collaboration revenue											$ 0	$ 100,000

Revenue Arrangements - Transact

Revenue Arrangements - Transaction Price Allocated to Future Performance Obligation (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 02, 2016
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 110.8
Spark License Agreement
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 2.3	$ 12.3
Spark License Agreement | License and Supply Obligation
Revenue from Contract with Customer [Abstract]
Remaining performance obligation		$ 7.1
Spark License Agreement | License and Supply Obligation | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2016-12-03
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period		4 years

Revenue Arrangements - Schedule

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Contract liabilities:
Deferred revenue, beginning of period	$ 16,354
Contract liabilities, beginning of period	16,354
Deferred revenue, additions	104,158
Contract liabilities, additions	104,158
Deferred revenue, deductions	(9,716)
Contract liabilities, deductions	(9,716)
Deferred revenue, end of period	110,796
Contract liabilities, end of period	$ 110,796

Related-Party Transactions - Na

Related-Party Transactions - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Founders
Related Party Transaction [Line Items]
Consulting services expenses	$ 0.1	$ 0.5	$ 0.2

Collaboration Agreements - AskB

Collaboration Agreements - AskBio (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Operating expenses	$ 73,418	$ 59,132	$ 65,925
AskBio License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Operating expenses	$ 3,800
Cost share percentage	50.00%

Collaboration Agreements - MIT

Collaboration Agreements - MIT (Narrative) (Details) - Spark License Agreement	12 Months Ended
	Dec. 31, 2020USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	$ 0
MIT
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement	$ 400,000

Collaboration Agreements - Shen

Collaboration Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments	$ 7
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing ImmTOR platform	$ 15

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Holiday [Line Items]
Valuation allowance, deferred tax asset, increase (decrease), amount	$ 16,300,000	$ 14,600,000
Income tax holiday term	10 years
Federal operating loss carryforwards	$ 234,500,000
State net operating loss carryforwards	228,500,000
Research and development credits	7,632,000	$ 6,586,000
Net operating losses available for use through 2028	36,500,000
Unrecognized tax benefits	0
Accrued interest related to uncertain tax positions	0
Domestic Tax Authority
Income Tax Holiday [Line Items]
Operating loss carryforwards, carried forward indefinitely	99,600,000
Research and development credits	4,900,000
State and Local Jurisdiction
Income Tax Holiday [Line Items]
Research and development credits	$ 3,500,000

Income Taxes Reconciliation of

Income Taxes Reconciliation of Income Taxes (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory U.S. federal rate	21.00%	21.00%	21.00%
State income taxes - net of federal benefit	5.80%	6.30%	7.00%
Permanent items	(2.90%)	(2.10%)	(0.50%)
Research tax credits/other	1.00%	1.10%	1.60%
Valuation allowance, net	(23.60%)	(26.30%)	(29.10%)
Stock Compensation	(1.30%)	0.00%	0.00%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes Deferred Tax Asset

Income Taxes Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets
Net operating loss carryforwards	$ 63,678	$ 71,270
Research and development credits	7,632	6,586
Stock-based compensations expense	2,919	2,516
Other expenses	788	812
Deferred revenue	26,699	4,468
Operating lease liability	2,884	102
Patent costs/amortization	5,379	5,111
Gross deferred tax assets	109,979	90,865
Deferred Tax Liabilities
Depreciation	(11)	(57)
Operating lease right-of-use asset	(2,991)	(82)
Gross deferred tax liabilities	(3,002)	(139)
Net deferred tax assets before valuation allowance	106,977	90,726
Valuation allowance	(106,977)	(90,726)
Net deferred tax assets	$ 0	$ 0

Defined Contribution Plan - Nar

Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Vesting period		4 years
Employer contribution made	$ 0.1		$ 0.1	$ 0.2

Subsequent Events (Details)

Subsequent Events (Details) - 2020 Sale Agreement - At-the-Market Offering - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended
	Aug. 31, 2017	Mar. 05, 2021	Dec. 31, 2020
Subsequent Event [Line Items]
Sale common stock	$ 50
Issuance costs			$ 0.6
Subsequent Event
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		4,119,613
Share price (in dollars per share)		$ 4.59
Sale common stock		$ 18.3

Uncategorized Items - selb-2020

Label	Element	Value
Accounting Standards Update [Extensible List]	us-gaap_AccountingStandardsUpdateExtensibleList	us-gaap:AccountingStandardsUpdate201409Member